If they released two new premium phones(probably unlikely) it's conceivable that one might be quite large, but if the premium end (ie. not the C range) comes in only one size, I'd be very surprised if it got much larger than 4.5"-4.7" or so.
There's no "correct" size of course, it's all preference, but get into 4.8"+ territory and there's a lot of people who flat won't like it. Apple aren't Samsung, preferring fewer products with wide as possible target audiences. Unless it's virtually without bezel, 4.8"+ would alienate a lot of people (not me necessarily).
I used to really want a much bigger iPhone but the Mini pretty much satisfies my bigger-screen-on-the-go needs as it fits into my coat pocket. I'd definitely like a wider screened iPhone along the x-axis though. I always had this completely unfounded pet theory that, had the screens been available cheaply enough and in the right time frame, battery permitting, Apple would never have gone 16:9 and stuck with 3:2 and a larger 1920x1280 screen, a pixel doubling of the 4's 960x640. As much as I love my 5(now 5S) 1136x640 has always felt like a compromise to placate the limitations of the os and deliver a then much needed size increase on the old 3.5" screen.