Imo, 5" screen size is the sweet spot now. Once you start using a 5" phone, which take me few days to get used to, coming from 4.3". You can never really go back to smaller screen, especially iphone 4 / iphone 5. They will be too small.

but of course, this is all up to personal preference and taste. Hand size might be a deciding factor as well.

If the rumours are real, 4.7" and 5.5" iphone, it will be a big dilemma.
Perfect screen size, but small battery, or big ass screen but good battery size.