The current crop of MS tablets seem to cut in on the laptop form factor. The idea behind a laptop is portability while still providing desktop capability. Since a tablet only allows a lower level of input, in my opinion it should be sized accordingly, since most users won't need the functionality of a laptop.
Approximately, the current computing form factors are pda, laptop, and desktop. I feel that desktops and laptops have fulfilled their roles quite well, but pda's are just too small to provide any real functionality. A tablet the size of a jr. size legal pad (5" x 8") would provide substantial surface area for note taking and other quick tasks that don't require a full fledged computer. As previously pointed out, a tablet for graphic designers would essentially be for graphic designers alone, since the form factor is too great for the input type. I couldn't stand using a laptop as a tablet because it would simply be too bulky. Only a graphic designer needs a bigger tablet style input device.
A good tablet again should be approx. 5x8 in. Other useful features would include:
write to the edge screen (no bezel)
3/4 in thick or less (this will make it hard to fit the innards, but is necessary for the form factor)
changeable orientation
Other details are a matter of implementation, but I'm presenting these ideas to describe a more usable form factor. laptop sized tablets and pdas are just impractical.
