I just recently bought a Planar 15" that I am using for IL-2 right now with WBK's 2.1 profile.
I'm finding that with the more screen area/buttons to push, the more attention that you have to expend in finding what you want to select and the more chances of missing something in the main sim window.
Now, it might get easier in time to go right to what you want, and I would still recommend get the biggest you can afford, but realize that it most likely would be best use just to make the buttons bigger and not put alot most stuff on it.
I have only had it a week now so take that with a grain of salt, but the busier and smaller the selection area, the more you have to turn and focus on the touch screen to hit the right spot
