Most of the recent leaks involving the new iPhone suggest that the screen will get larger, but the exact size is still unclear. Several reports indicate that Apple is testing out a larger, 5.5-inch iPhone while other sources point to 4.7 inches. If either rumor is correct, the larger screen may drastically change things for the iPhone going forward, with the biggest implication involving its resolution.
Apple could keep the same dimensions as the iPhone 5 models, but a larger screen operating at the same resolution would mean the phones would no longer be within Apple’s Retina capability. While this change would be the most simple because it does not require changes in apps to function on a larger screen, the loss of Retina would mean the display would be less crisp and look more like pre-Retina displays such as the iPhone 3GS and older.
The more likely scenario is that Apple would change the phone’s resolution just as it did with the larger screen on the iPhone 5. When Apple switched to the 1136 X 640 resolution in the iPhone 5, app developers had to release updates to change resolutions. While apps without the update will continue to function, those apps will only use part of the screen.