Could real giant humans exist?
Mythical giants and allegedly real giants are known in many cultures. A tentative definition probably valid at the time of writing is a humanoid creature significantly larger than the tallest known human without the weaknesses associated with human giantism. There are limits on how large a human like creature could be and still remain human, but there seems no generally accepted value for this limit. One approach to this is to look at reports of giants and skeletons of giants to see what the limit might be. Such a study suggests giants, if they exist, are between 9 and 15 foot tall, possibly as large as 20 foot tall and possess characteristics suggesting they are humanoid but probably not human.
It is necessary to consider “evidence” produced by creationists and other religious extremists wishing to prove their scripture the inerrant word of their god. Creationists are as valuable as holocaust deniers are to holocaust researchers: even though their basic premise denies the evidence against it, they pose valid questions which can develop into fruitful lines of research. Evidence that events in the Bible were “true” does not, however, imply the Bible is the “inerrant word of God” : the latter assumption is an article of faith not a logical consequence. Creationist evidence may have to be considered but given low weight.
The involvement of militant religion, militant atheism, skeptics and conspiracy theorists makes this field a highly charged area of research where at present no firm conclusion can be drawn. There is also a tendency for the “best” evidence to vanish hence making it no more reliable than other reports.
What is a giant?
A giant is a human or humanoid significantly taller and broader than the average or tallest human, possessed of human level intelligence, and often of great strength. If a true (human) giant is defined as a very large man or woman normally proportioned but with no growth abnormalities and looks at the range of human sizes there are many professional wrestlers around seven feet tall, the Masai of Africa are exceptionally tall, and Angus MacAskill , who died in 1863 at the age of 38, and is regarded as the worlds largest true giant  , stood 7 foot 9 inches tall and weighed about 500 pounds in adult life. MacAskill could lift a one ton anchor to chest height but crippled himself when one of the flukes of a 2,000 to 2,700 pound anchor he was carrying caught in one of his shoulders.
Tentatively any human or human like creature that is much taller than 8 feet will, here, be considered a giant. Of course such a body will require extra muscle for day to day activities so evidence of exceptional strength is a weak indicator of true giantism.
How big are giants?
Worldwide, remains of humans up to 25 feet tall have allegedly been found  though in the latter case the evidence is based on teeth and jaws, a form of evidence that has proven unreliable with the teeth of large animals such as elephants being mistaken for human ones. In the case of Australian finds of course elephants are an unlikely source of confusion but the continent has apparently had large animals in the past. The report of extremely heavy stone artefacts near Bathurst Australia is not conclusive as neanderthal man, shorter than today's average, is considered to have been exceptionally strong by modern standards. It is also possible the artefacts were oversized copies used for ritual.
Assuming no misidentification the upper limit to the size of a giant is around 25 feet. There is a problem here because if proportioned like normal size humans the giant would be very heavy. If we take normal weight for a five foot human as 100 pounds a person twenty foot tall would weigh about 6,000 pounds ( assuming the square-cube law that weight varies as the cube of their height and the muscle cross section varies as the square of their height ) and, like neanderthal man would need stronger bones and more efficient muscles. And perhaps larger feet to spread the weight (Hello Bigfoot!!) . True Giants of 8 foot stature tend to weigh around 500 pounds and a 16 foot human would weigh about 4000 pounds.
In 1991 a grossly obese man, Walter Hudson, who weighed around 1190 pounds, possibly the heaviest man on earth at that time, died in New York Using the square cube rule above this would correspond to a True Giant of around 11 foot 5 inches. Since it took Hudson an hour to walk to the toilet six yards away we would have to take the unconfirmed reports of 9 foot three inch giants as indicating a limit of ten foot for human giants.
At much over ten foot other changes would need to be made which would make them not human. For example a thicker skin to prevent the pressure of their blood bursting the skin. Also basic anatomical facts, all else being in proportion, would mean they could not breed with humans, thus contradicting claims in the Bible.
This means that if Giants of 20 feet or more existed they were not human and would probably have been very broad with stumpy legs, more like those of elephants than humans. The fact that elephants can stand on their hind legs makes the square-cube law argument a little weaker but it is still a potent argument against the existence of giants over 10 foot tall.
Hoaxes and Speculation
The possibility of hoaxes can never be ruled out in any single case, for example  is a set of hoax images originally used for a photoshop contest to create fake archaeological anomalies , but the worldwide occurrence of such remains and reports going back centuries would need a dedicated multinational band of hoaxers operating over a long period of time or independent hoaxers all reporting finds consistent with each other and none slipping up and leaving traces. Somehow the existence of giants seems more likely. But this is not conclusive. The giant could simpley be an archetypal figure.
One interesting point  is the giant skeletons found that had double rows of teeth and/ or six fingers and toes. If these finds can be verified it begins to look like they were humanoid but possibly not human, though they may have been a variety of human.
Some seven foot skeletons found in Pennsylvania had horn like protrusions above the brows, and human with horns have been observed even today. Neanderthal man had very pronounced brows and there may have been cultural or evolutionary selection for this trait in Pennsylvania, though this is pure speculation and not helped by the fact that the skeletons seem to have vanished.
There also seems to be a tendency for these giants to have red hair, a feature often associated with Sasquatch, Bigfoot and the Orang Utang. It would be speculating well beyond the data to say that the Sasquatch is a degenerate descendant of giants that once roamed the USA and were banished to the wild lands by Homo Sapiens, but the possibility must be entertained.
An additional barrier to belief is that some of the giant footprints found have been dated millions of years in the past, to a time when humans had not yet separated from primates. While this may strengthen the case for the alleged 123 million year old map of the Urals  that emerged a while back (though again the case tends to evaporate on close scrutiny) it does not endear the data to scientists or to those religious believers for whom everything happened at 9:15am on a sunny day in 4004BC.
The examination of reports of remains of giants indicates that:
Great care has to be taken in assessing individual reports.
There may have existed a variety of human to 25 foot tall but no higher with red hair and 12 toes and 12 fingers.
The reports are worldwide and cover a vast period of time.
Given the unreliability of modern photographic evidence and the prevalence of hoaxes other evidence must be examined.
Legends and a sighting
Australian Aborigines claim, in their ancient folklore that Australia was settled by giants and several groups of men before they arrived and anthropologists say the Aborigines were not the first inhabitants of Australia but at their predecessors who were akin to New Guinea natives [3 but no sources cited ]. There is some disputed evidence that the kimberley region of Australia was occupied by a relatively advanced civilisation which the ancestors of the current Aboriginal Australians wiped out. Giant footprints and skeletons have been found in Australia but these finds are subject to the uncertainties mentioned above.
Some American Indian tribes have legends of encounters with tribes of giants who were there before the Indians arrived. Often the giants are red haired. One source claims Boudicca was a red haired giant.
There have been reports of True Giants in the Solomon Islands, East of Papua, and while these look promising they are best regarded as subjects of another story
A living giant was seen in Buffalo Mills, Pennsylvania, on August 19, 1973. He was at least nine feet tall and, dressed in strange clothing, which appeared to be made of some sort of shimmering material, strode down the main street of the village. He looked at the startled townspeople in a dark, penetrating way and then loped off casually into oblivion. Clearly given eight foot tall people exist this is not implausible but seems to be related more to the strange visitor class of sightings.
There are alleged instances of Roman armies encountering giants and usually being defeated  but in a quick cross check on a couple of accounts of battles Wikipedia did not mention giants. To assess these accounts properly would require reading original accounts from the classical authors. The same site gives details of extremely tall people in various parts of the world at various times, and seems to establish a limit of less than nine feet for human size.
The Classical and Ancient World
The Biblical Goliath would have come in at around 9 foot 6 inches (though the size of the cubit may have changed with time) and the Roman Enperor Maximus of Thrace was said to be around 8 foot 6 inches, like Angus MacAskill. The Emperor Jovian was supposedly so big it was impossible to find an imperial robe big enough to fit him and Suetonius describes the Emperor Augustus as embarrassed by being short at 5 foot seven inches, just below the original height requirement for the British Police.
While we have seen populations getting taller and larger once childhood nourishment improved, the Ancients believed we were getting smaller. Pliny considered this the result of moral collapse reducing the strength of semen.
Pliny mentions a corpse 69 foot long on Crete, an island allegedly populated by liars, while the remains of Orestes came in at 10 foot 6 inches. In Augustus time a couple of 10 footers were buried in the gardens of the Historian Sallust. In Pliny's own time the tallest person was Gabbara at 9 foot 9 inches, who was imported from Arabia. An impressive Jew called Eleazer was given to Tiberius and allegedly stood 26 feet three inches high: So much for the theories of Aryan Superiority.
Of course some of the accounts, especially those of corpses, can be put down as misidentification of ancient animals, and there may well be a degree of exaggeration like the tale of the fish that got away and it is not easy to relate ancient units of length to modern ones, but the plausible cases recorded when the Giant was living, suggest the human race may have been larger then than now.
Most of this section came from Classical Corner, Fortean Times, August 2012
Every one of the book-worshipping religions whose holy scriptures mentions giants has a vested interest in proving giants existed. In the case of Christianity this is a double edged sword since some of the evidence cited would point to giant humans being around millions of years ago. Creationists get round this by claiming that humans and dinosaurs coexisted, as in The Flintstones, and in his book Idiot America Charles Pierce mentions a creation science institute that harbours a stature of a dinosaur with a dressage saddle. This appears to be patching the theory a little too much. On the other side are militant atheists, mostly working out childhood issues with religion, who are determined to discount any evidence on the flimsiest grounds. Then there are academics who have invested their life in the standard model of the remote past and feel their lives would be disrupted if the existence of giants were proven. Indeed if the existence of intelligent language competent giant humanoids were proven then much would have to be rewritten. With all these factors it is not surprising that the study of the possible existence of prehistoric giants is a somewhat taboo field.
Investigating the possibility a race of possibly non human giants inhabited parts of the earth thousands or even millions of years ago is a complicated and politically high risk undertaking. Hoaxes and self deception abound and solid evidence tends to vanish. In some reports solid evidence in the form of buildings has been found only for the sight to be purchased by persons unknown and then vanish from the record. The research that needs to be undertaken is a detailed examination of the evidence available, debunking as much as possible (though the debunked material may give valuable insight into the psychology of individual and cultural self deception).
One intriguing point in this investigation is that the giants mentioned often have double rows of teeth plus twelve fingers and twelve toes. Some legends also imply communication between giants and humans suggesting the giants, if they existed were not remnant hominids like gigantopithecus who may not have had any capacity for language.
A rational case may be made for the existence of a race of giants and a rational case made against the existence of a race of giants. All it would take to prove the existence of these giants would be ONE incontrovertible corpse or skeleton. However I recall the Piltdown Man Hoax which fooled all the experts for about 20 years. I also note that the "solid evidence" so often cited tends to disappear when closely examined.
I speculate that the giants are part of what John Keel called “the Phenomenon” and intended to keep us guessing and distract us from.... something.
As usual much more research is needed. As usual it is unlikely to get done.
 The link to which this refers has vanished from the internet. It was a hoax, or at least using hoax pictures 
 Debunking  The video to which this refers has been removed by the creator
 This link has vanished, It was a “prove the Bible” site and saw an atheist conspiracy by places like the Smithsonian to suppress knowledge of giants. A fine example of the pitfalls that befall the researcher.
 Another hoax
Efeyas has also written on Mysterious Human Remains