[Benjammer]

Agreed that size thing doesn't really have justification anywhere, but w/r/t the second part of your comment:

What about the fact that Hydrogen, Oxygen and Carbon are the basic building blocks for all types of life that we have observed so far on Earth, and they are also 3 of the 4 most prevalent elements (H, He, O, C) in the observable universe in general?

Sure, people have theorized that there could be Silicon- or Germanium-based biochemistries out there that work similar to Carbon-based life, but it's probably more likely we're going to find something closer to a Carbon-based biochemistry than not, just given the distribution of elements out there, right?

I think we can still talk about statistical probabilities without having actual observable data on a non-Earth life form, can't we?