Perfect size for me is my 6S Plus' 5.5. Hope they keep the screen size because I'd hate to go back. I'm actually typing this on my 3GS, which I recently downgraded to iOS 4, it now flies like it should! It's sort of fun seeing how things were with 3.5" screens and it's quite usable but some of that might be nostalgia. My 6S Plus is slightly more inconvenient to carry around but it's MILES more enjoyable to use. I've been using my 3GS and 5 most of today and periodically going to the 6S Plus and there's no comparison.