If Apple goes for that 0.3 increase, 5" iPhone will probably be around the same size as the HTC One M8 or M9. The iPhone 6 at 4.7 is practically the same dimensions as the HTC One M7 but thinner and rounder on the edges. I used 4.7 & 5" smartphones in tandem and felt no real difference. It took me awhile to get accustomed to 4.7 coming from 3.5-4", but now I want to push my limit to 6". Just a matter time of getting used to the screen sizes. After two years with 4.7-5", 4" or under looks small. Like a baby's phone. If I go 5.5-6", the 5" I am currently used to will start to feel small.
Apple can keep it at 4.7 and nobody will really miss out on the 0.3 increase. Apple can go 5" and nobody will really feel a difference coming from an iPhone 6. An inch or more increase, and you will feel a dramatic difference. Less than half an inch, nothing except a few mm wider for a slightly roomier keyboard in portrait mode.