Aurelion Sol SHOULD be the same size as Dragon.
But no... he's about the size of a human
But no... he's about the size of a human
His head alone is the size of a human.
You're actually still not thinking large enough. Those little balls of light in his hands, yeah, those are STARS! Really the best adjective would not be large, huge, gigantic, or even colossal, but instead 'galactic' would probably be best. The League uses their magic to scale champions so that their respective size is not a overwhelming factor. I believe he is by far the largest so far though.
Yordles should all be the size as Rumble (the little guy that shoots out when you kill him, not the whole mech).
Nautilus is massively bigger then most humans.
Malphite is the size of an actual mountain
Maokai is a freaken tree.
TL;DR: sizes are not accurate in the game so that it is actually playable