My Tablet use started with an iPad2, which was kind of ok on the knee. I switched to iPad3 and threw it off the cockpit, as it was too heavy for the kneeboard. Now I am at an iPad Mini Retina, which is almost perfect: it does have enough pixel to be nice to look at information and it fits in the knee board in a rented plane on on the yoke in my own. So 7" is my favorite, but it should definitely be the retina display.