If I read this correctly, the argument is that a) intelligent life could be anywhere from 50kg (a chimp) to 500kg (a small elephant). Since humans are at the low end of that range, and assuming an even distribution across the range, then most other intelligent life is physically larger than us. (There is a tangent relating to population density in there, too).

It's a clever argument. But to be honest I don't think we understand enough about how intelligence arises (consciousness) to really put a bounds on the physical size of species. For example, there is no evidence that any dinosaur was intelligent - why not? Additionally, it seems likely to me that over time humans will virtualize and inhabit different kinds of bodies, probably smaller ones, and that this could be a typical progression for intelligent biological species that don't annihilate themselves.

But most importantly, there doesn't seem to be any real utility to the argument, even if it were sound. I mean, let's say the author is right - so what? Okay, the average intelligent alien is bigger than us. What does that have to do with anything? How does it help us contact them, visit them, understand them? The only application I can think of is as material for certain kinds of science fiction authors' in-story expositions.