big and small
SMALLER
Selling our iPad and iPad 2 if a ~8" and Retina full size iPad come simultaneously. Our kids love the iPads and would be fine with the smaller size, while I actually think the text would be MORE readable at that size with the same 1024x768 resolution but smaller area. It won't be retina DPI, but it will be much nicer to stare at than the full size iPad. The Retina display on a 10" is just a dream device to me.
BIGGER
I've wanted a bigger iPad since before the iPad 2. 15" would be pretty incredible. It will be a LONG time before something like that is desirable to many or practical, but it could rock pretty hard for many applications (music and video editing, watching content, drawing and painting, kids books and graphic novels, board games and other group activities, even document editing would start to be much more practical at that size.
What could happen first (in following years) is the MacBook Air could get a multitouch screen along with MacOS X gaining more and more of the sensibly transferrable features from the iPad.