I have played with both 7 (Nexus) and 10 (ipad 3). In many cockpits 10 is too big, so have standardized on 7. The screens are not visible in bright sunlight so I have not mounted them on the panel etc. I find that I have to move them around to get the best viewing angle which changes depending on sun position etc. I fly low wing aircraft so this would probably be less of an issue in high wing.
Rod1