4,7" no doubt. 5" is maximum screen size if you care about one handed use. I believe that apple did right decision about the screen size. 4,7" is perfect size for a modern smart phone.
I am very happy with the 4" screen on my 5s....one of the main reasons I like iphones. Unless there is a must have feature I will just stick with the 5s.
If I upgrade, I will pick the smallest of the new phones.
My Plan as a current Samsung Galaxy S5 owner that converted from an iPhone 5S in April.
1. Watch Keynote and see what iPhone 6 is all about
2. Run iOS 8 on my iPhone 5S for a few weeks
3. Determine if any of the new features are anything special or tangibly valuable to me.
4. Visit the Apple store and fondle the iPhone 6 several times.
5. Get iPhone 6 or pass on it based upon my impressions