iOS & Apps will be just fine on a larger display and higher resolution: How iOS looks like on a 4.7-inch display
Apple may be working on a larger display with a resolution of 960 x 1704 giving the iPhone 6 a display density of 416 PPI on a 4.7-inch iPhone 6 and 356 PPI on the rumored 5.5-inch iPhone 6. In the event that either of these display sizes end up on the production line, sources say that most of iOS will simply be enlarged to fit a bigger display.
iOS And the Apps will be just fine on the new 4.7-inch display size and the new resolution that will make it have the incredible 416 PPI. I have already blogged that there will be no problem with most of the Apps, they will just be good enough until the developers has updated them to take advantage of the new resolution.
Hopefully Apple has added some new controls (gestures) in iOS 8 that makes it easier to navigate the UI on a larger display too, and moved buttons to the bottom instead of the top and so on. That's the beauty with having full control of both software and hardware.