No one buys tablets by the square yard, though. Lots of people seem to say that a 10" tablet is inherently better than a 7" tablet, but I think the market is still far too young to really know the winning form factor.
Anecdotally, I find that the iPad size is great for a table/lap, but less great for holding, while I love the size/shape/weight of the Kindle2 for reading. I don't think any particular size will win out.