Contrary to everybody's belief that they'll bump up the size, I think they'll put in a flexible screen before increasing the size:
Samsung said to be bringing flexible 5.5 inch display to CES
5.5 Flexible screen sounds like the best option, considering there have already been horror stories of some people cracking their screens (and some of the drop tests showed cracking right away).
I wouldn't mind if they went that route first, and then figured out how to turn the flexible screen into a bigger screen, while using that flexibility to keep the form factor where it currently is.