It‘s always hard to draw the line for realism in science fiction. But it’s not only about physics and stuff, no! Biology also has some nastry tricks up its sleeves.
Up to which point it still counts as a healthy endeavor to try and be as realistic as possible in hard sf, I’m never sure. But I certainly have some stretch goals in that direction.
While I was reading the anthology The Oxford Book of Modern Science Writing yesterday, I came across this fine essay “On Being the Right Size” by the geneticist and evolutionary biologist J. B. S. Haldane. Size, body size, eye size … well, see for yourself. Just follow the link to the essay’s online version. In case you’re in the business of creating alien species, it gives you a hint about how many wonderful opportunities there are to goof up royally.