I think 4.3" would be the sweet spot. 4.7" seems large. 4" seems a little small.
Here are a couple of comparison pictures.
The first compares the 5s to the 6 and the Z1 compact which has a 4.3" screen
The second compares the 5s to the 6 and a Samsung Galaxy Alpha which is a 4.7" screen like the iPhone
As you can see, the Z1 compact is comparable in size to the 5s and the Alpha, whilst bigger than the 5s, is quite a bit smaller than the 6.
Basically, I think there is plenty of scope for Apple to reduce the size of the phone whilst keeping the 4.7" screen