I really wanted a 13" but when i finally go hold of one i couldn't believe how unportable it was, picked up the 11 and within a few mins it felt natural. Haven't looked back since and love it even more every time i use it.
I've used my ipad1 as my computer since launch. I do have an (late 2006) 17" MBP that sits on a desk, but really rarely "need" to use it. I'm needing to add a computer to the house and while my original intent was to build a Pro hack (hey, I'm a poor single mom!), I need a portable solution too for school stuff.
So I decided on an Air with a matte display for home. I've been sitting here at home with a ruler comparing the sizes to my iPad and thought the 11" was what I wanted. Last night I made it to the apple store and yup, 11" won. The 13" just looks HUGE after the iPad. I had two prior 12" laptops (Sony r505 and iBook) and I couldn't believe how much bigger it looked. It has to be because of the 16:10 screen vs 4:3??
Anyway, do go play with them. I'm happy to have decided - but now I have a few weeks wait for my financial aid to roll in. I'm not looking forward to playing musical computers with my daughter as we both try to do online classes!