Depends. If I keep my Pre+ or they introduce a tiny phone that shares resources with a tablet... probably the 7". But the kindle is the perfect size as long as they give it decent resolution. I think they should all be 720p, but certainly 1280x900 (or 1024) should be the minimum resolution to be my primary screen. If my next webOS phone is closer to 4" then I'd want the bigger tablet.
My netbook is 13.3" and 720p, and I really don't want anything smaller for extended use with these tired eyes.