|Cover via Amazon|
With the smartphone we are already in the four and a half inch territory. We are almost there. Amazon cracked the 7 incher with the Kindle Fire and Google nailed it with Nexus 7, and Apple is scrambling to get in there.
10 inches is almost Macbook Air territory. Things start getting a little fuzzy around there. And if touch is not to stay put at smartphone and tablet levels, if it is also to be part of the PC experience - Hello Windows 8 - then the size thing becomes even more important. It is just about the size. Otherwise it is pretty much the same thing. Which probably means you don't need them all. I think it is possible to get by with two.