I'm betwixt and between on the size issue...
Typing anything on my 5S remains a singularly frustrating experience - in the words of the SO, it works fine if you have toothpicks for fingers... So more real estate would be useful >> BUT, the promised 3rd party apps for typing/writing that are due in iOS 8 will probably fix just about all problems in this regard?
More real estate would then only(?) be a plus in terms of web-browsing/youtube etc? >> but I have an iPad for that... So not necessarily convinced...
Not sure how much of an irritation the bigger form factor might be i.t.o. carrying it around, and the all-too-forgotten issue(?) of not being able to reach all of the screen with one hand... Mentioned this before, but having had a phone with a stylus before - one doesn't really appreciate how much you actually rely on being able to control it single-handedly, until that's no longer an option...
So would also be interested to see what happens if the smaller models are retained.
Having said all of that -- would probably upgrade to the next biggest size, assuming they bring out two larger versions. But cannot see myself going for a phablet of sorts... [but who knows...]