I'm not really sure how it works. I've stayed in the Wonder tower in a small room, and the Magic tower in a large room. The regular rooms we've had in Dreams have all been the same size, which to me is average (but larger than most of the rooms in GCH). I was given a *teeny* room in the Magic tower last Nov. but I didn't accept it and was moved to a giant room in the Dreams tower, which was actually part of a King Suite so I don't know if you can compare that one. I had heard that the original tower (which I think was the Magic tower, where the lobby is) had the largest rooms, but I have no idea. I've also heard that some of the rooms in Wonder are quite large, but definitely not all of them as the 2 times I've stayed there they were small. Dreams is the newest tower and supposedly has the smaller rooms, but I really couldn't tell a difference between those and the usual ones we get in Magic.
More important than the square footage, IMO, is the layout. Some have an extra dressing area and some have a larger bathroom. Some have extra (wasted to me) space as a sort of entryway, which I'd much rather have in the room or bathroom. We had one room that had a sort of dressing area on one side of the entry door and the bathroom on the other, which gave you a lot of space and options for using the restroom and getting ready in the morning--important when you're a family of 5. I think part of the difference in layout has to do with whether or not it's a connecting room.