If they want to remain faithful to their traditions, they should keep also 4 inch smartphones. Even though after few days with my iPhone 7 Plus (5.5 inch) I find my wife's iPhone 6S (4.7 inch) too small.
Depending on which, if any, rumours you choose to believe we could see a bezel-less 5" screen in the same overall package size as the current 4.7" screen model. I'm not holing my breath, but that would work well for me.