The screen size comes down to a tradeoff between portability and productivity. You can get away with a surprising amount on a 10 inch screen - in fact, it covers just about everything you would want to do "on the go".
The next step here would be when you come to a desk that has your keyboard, mouse, and big screen and the tablet interacts with them simply by being in their vicinity. That hasn't happened yet but I bet it will and the whole debate about Tablets and PC's will be reduced to meaningless semantics.
> your keyboard, mouse, and big screen...the tablet interacts with them simply by being in their vicinity
Bluetooth and Intel's wireless display standard [1] are here already.
Support for these technologies in common devices is another matter...especially considering that wired devices are cheaper, don't suffer from RF congestion, and don't need their own power supply (at least in the case of keyboard and mouse).
The next step here would be when you come to a desk that has your keyboard, mouse, and big screen and the tablet interacts with them simply by being in their vicinity. That hasn't happened yet but I bet it will and the whole debate about Tablets and PC's will be reduced to meaningless semantics.