There is a myogenic gene that prevents people from growing bigger than a certain point. That means that everyone has a biological maximum of muscle size.
This actually makes it very useful for humans, since after a certain size of muscles, more muscle on top of that makes for dead weight instead of efficient muscle.
Also it is considered very unattractive by many to be over muscly and so a biological cap is very good for this.
That said, there are people without this myogenic gene and therefore can grow indefinitely. These people often become muscle builders. They are not however any stronger or faster than people with the gene (because of all of the dead weight muscle) and that is why you do not see muscle builders doing boxing or professional sports.
To answer your question more simply: the power of a man is limited to his height and his myogenic gene.
The taller a man is, the stronger he is because there is more space for efficient muscle mass (not dead weight muscles).
After a certain point of height, the human skeleton will not work because there will be too much pressure on the bones. Therefore, it will be just below this height that a man will be the strongest. That would be the uppermost limit of the strength of a human.
Unless of course some humans evolve so that they can grow even taller. Then that would be the limit (if that's even possible).
I hope that answers your question and that it wasn't too complicated. Good luck!