We humans have a tendency to build a hen out of a single feather. We are often fabricating a story out of a few words. When it comes to these Giants, we have the Bible that tells us about them. I will share what I know it says:
(Genesis 6:1-4) 6 Now when men started to grow in number on the surface of the ground and daughters were born to them, 2 the sons of the true God began to notice that the daughters of men were beautiful. So they began taking as wives all whom they chose....... 4 The Neph′i·lim were on the earth in those days and afterward. During that time the sons of the true God continued to have relations with the daughters of men, and these bore sons to them. They were the mighty ones of old times, the men of fame.
So here we see that Angels put on human bodies and married the human women. (They had observed that humans had sex and were tempted to try this new feeling?) It is interesting to note that they married before the sex - "they took wives" the verse says.
This was so wrong in Gods eyes. Actually Jude compared these acts with the fornication that later took place in Sodom and Gomorra.
(Jude 6, 7) . . .And the angels that did not keep their original position but forsook their own proper dwelling place he has reserved with eternal bonds under dense darkness for the judgment of the great day. 7 So too Sod′om and Go·mor′rah and the cities about them, after they in the same manner as the foregoing ones had committed fornication excessively and gone out after flesh for unnatural use, are placed before [us] as a [warning] example by undergoing the judicial punishment of everlasting fire.
One of the results was that the babies that was born to these "forme angels" were Giants. Nefilim the Bible calls them - that is translated possibly meant - “The Fellers,” that is, those who cause others to fall down. They were violent and caused terror on earth. This is the reason that God descided to make the flood come to wipe the earth clean.
These transformed angels did not die in the flood. They returned to their spiritual bodies. But we can read in the Bible for a last time about them:
(2 Peter 2:4, 5) 4 Certainly if God did not hold back from punishing the angels that sinned, but, by throwing them into Tar′ta·rus, delivered them to pits of dense darkness to be reserved for judgment.
The Bible does not say anything about how tall these children grew. Nor any details about them than this.