[quote=Theodorable]
The class Aves (commonly called avians) are iffy. The reason I say that is because avians are one of the few biological classes of animals that has definitively shown a genetic predisposition to favor smaller bodies. Insects, it has shown are able to grow as large as the amount of oxygen they can successfully filter to their organs. Ichthyology has shown that evolution favors size based on principle and demand. Avians seem to fill a niche specifically designed for smaller creatures. The same can be said of the order Rodentia.
[/quote]

But limiting such Earthly concepts of life upon alien lifeforms seems illogical, doesn't it? Who's to say bird-like aliens could not exist that are tremendous?

And what does size have to do with anything anyways? :P