I think 5.2" with similar bezels to the current would be perfect. Screen size does not mean the the device will be that much bigger than current.
Also, judging the size of a phone from a screen number is ridiculous. Until we can hold it in our hands, numbers mean nothing. Design is everything. Screen size means nothing.
Everyone here is freaking out over a possible increase to 5.2, but go look at the iPhone forums and look at the complete freak out of moving up to 4.7 if you want to see some really distraught people. I used to think 4" was the limit, but now that I have actually given a larger phone a chance, I want a little bigger for better media consumption.
My point in all this is: don't knock a phone that does not yet exist on a number. After it is out, you can hold it and judge all you want.