I'm in the market for a new iPad, and if it is to be the Pro, it will most certainly be the 11 inch. What I love about the 11 inch is the smaller bezels making the device even more portable than the 10.5 inch (we were using minis for a long time at my house). My wife loves portability, but I love a larger screen. I tested the 12.9 inch once (this was when the size was first released) and despite having plenty of screen to use, it just became to much of a burden to carry around. It was unwieldy, heavy and uncomfortable after a while. We have a 15 inch MacBook Pro, so for any tasks that I will need a larger screen for, I will end up using the MacBook when not using the iPad.
The 11 inch I feel I can carry around the house with ease and it feels much more comfortable in the hands for long-term use. It's just my two cents, but unless I needed the 12.9 inch size screen for work purposes which I don't, I won't ever get an iPad that size for myself and my wife certainly wants nothing to do with it either lol.