Our Bodies are at the Midpoint of All Possible Sizes
The largest size we know is the size of the visible universe; the smallest size is set by the interaction of relativity and quantum mechanics, and it’s called the Planck length. Logarithmically, we are right in the middle. And we couldn’t be anywhere else, since if we were much smaller, we wouldn’t be made of enough atoms to be complex enough to have our consciousness. If we were much bigger, the speed of communications among our parts (which is limited by the speed of light) would be too slow. If there were an intelligence as big as a galaxy, it would only have had time for the number of thoughts most of us have in a few minutes. Complex consciousness is the job of creatures about our size, and this goes for intelligent aliens, too, if they exist.