Our bodies are designed for life on Earth. However, they’d be able to cope on a planet slightly larger or smaller than our own. If humans tried to land on a much larger planet, say Jupiter, they’d be reduced to pancakes by the tremendous force of gravity. If, on the other hand, the planet is too small, the size of an asteroid for example (even though, technically speaking, an asteroid cannot qualify as planet on account of its size), one leap and you’d fly into space.
In theory, you’d expect creatures that have evolved on a larger planet to be much stubbier and heavy-boned than earthlings, whilst those that have evolved on a smaller planet would tend to be much taller and delicate than us.
Besides the issue of gravity, however, there are other parameters you should take into consideration. Small planets, for example, cannot sustain an atmosphere and would, therefore, be uninhabitable.