Why are all these aliens roughly the same size as humans? If I had to play out evolution a billion times on a billion planets I would think most winners of the evolution race would be the largest animals. Also I would assume if they evolved on a planet similar to outs with a denser oxygen rich atmosphere they could be much larger than our largest dinosaurs were or even evolved in an environment like our oceans .aking them even larger.

Nah they are going to be bipedal and 5'9"