I saw the 11" in the Apple store and it was nice, it looked cool if you travel a lot (either mass transit, trains, planes) because of seating, etc. However, I could never use it as a main/primary machine.
IMHO, 13" is the sweet spot. The screen will also be a higher resolution than the 1280x800 of the old Macbooks. 13" form factor + Higher res screen makes it a winner, IMHO.
I agonized over this for a long time, do I get the 13" pro or air? In the end, I felt the most comfortable with the 15" MBP as my main computer, because I don't travel often, maybe 1-2x a year, I don't sit at Starbucks with a laptop and the only thing I may do is bring it to a relatives house. So the 15" with a dedicated GPU is what I went for.