After using the iP5 for about a month now, I'm quite happy with the form factor of it. I don't have any desire for a larger phone right now, although I reserve the right to change my mind as old age sets in and my vision goes.
After seeing some of the larger Android based phones, I really wonder about the general utility of having such a large screen. Yes, I suppose it would be nice for viewing movies and documents, but I actually still use my phone as a phone. Having to hold a large heavy slab next to my head to talk seems like going backwards. The iP5 right now just fits my needs, and I don't see much upside in having a larger screen for my usage. If I want to watch a movie or do some work, I'll use my iPad which itself is better suited to "document" work. A large phone just seems like a compromise between the two sizes, and for me handles neither phone or tablet work well.