I was playing around with some tablets at my local Fry's today, from the Galaxy Note 10.1 to a 12 inch ASUS windows tablet, and I can't realistically see myself using anything larger than a 7" device. My netbook (ASUS Eee PC) is 10.1 inches, and the idea of holding that screen in my hands doesn't seem practical. There's only so much you can do with your thumbs across a large screen while still trying to hold the device up, and one-handed holding seems like a burden.
It really shocked me, but the Nexus 7 can fit in just about any pocket I throw at it, including my basketball shorts! Anything larger would require a bag, which would diminish the whole portability concept.