Why are humans only so tall/large?
The question is a bit vague but I will take it to mean the following


  Why does it (the body) stop in terms of height or physical mass when it can still keep on growing?


The answer is physics, specifically the ability for a body of a specific shape and structure to support itself and move itself, followed by the energy requirement to feed all those cells. 

Strength of a bone scales with its cross sectional area, similar to muscle. Mean while mass scales with its volume. As a result, mass increases faster than bone strength. So the bigger your are, the weaker your bones become if all proportions are maintained. As a result for a certain design of animal there is an upper limit of how big it can become before it unable to support its own weight or able to move.

Next comes energy requirement. In general the more living bio mass you have, the more energy you require to keep it alive. The scaling isn't proportional but there is a general trend. And the more energy required the more food is needed, and given the habitat an animal lives in, there is only so much food around. And this places a limit on how big an animal can grow. An animal living in its niche has an energy budget.

Associated with energy budget, we meet biology and natural selection. Energy placed into growing, isn't energy placed into reproducing. All animals eventually die, either by predator or bad climate (winter, drought, even bad luck) . So it becomes a balance, given the amount of energy available, how much energy should be put into growth to give a more robust structure to survive vs how much energy should be put into reproduction so that the organism can multiply. 

Put too much energy into growing bigger, your probability of dying before reproducing increases as you take too much time growing before becoming sexually mature. Put too much energy into reproducing, upon sexual maturity your body is small and weak and your die before you spend much time reproducing.

The ultimate right answer is dependent on the animal's niche, the physical environment around it, its biology, and the predators around it.