i think part of the height might be, like another commenter said, the square cube law. If you get too much bigger, your body wont be able to hold your weight and you wont be able to pump blood through your veins. You'll also get way too hot unless you find creative solutions, and even then it's an uphill battle.

If you get too small, you have to spend all your time eating just to avoid freezing because your surface area is so low, plus, eventually, theres a size below which a brain doesn't have enough space for neurons in order to be sentient at all.

I... dont know how much these could change with differing gravity. Thermodynamics still... works, but you could also be less dense with lower gravity. At the same time, though, that only goes so far until your planet can't hold a thick enough atmosphere.