7"

Good size for personal viewing, still share with a friend or two. Small enough to slip in a back pocket or jacket pocket for portability. Still small enough for picture/video taking (with a decent camera)

16x9

Good for movies, games, etc. Reading isn't an issue for me in either portrait or landscape. Balance isn't a problem for me either. I've always wondered why 16x9 is fine for reading websites, but ebooks need 4x3? For some reason, everyone assumes that ereading is optimal in a format similar to traditional books, I read primarily in landscape.