It is extremely likely the 5s will become the 6c later this year. They will very likely continue making another iteration of a new 4" model as well and at most might increase that to 4.3" if they could keep the entire size of the phone around the same size. This will be the phone for people that feel one handed use is paramount and essential. It makes very little sense for Apple to abandon the very successful small display market where they dominate.
A lot of other people don't really care about one handed use at all. People that spend far more time with apps than just a quick 10 or 15 second glance likely use even the current iPhone with two hands. These people would love to have a larger display since their entire user experience would be greatly improved. Many apps work better or only in landscape mode which requires two hand use. For this group Apple will likely make an additional model around 4.8" to 5". I am not sure if in 2015 Apple would put the larger iPhone in a plastic case and sell it cheaper like the 5c but that would give Apple 4 distinct models. 2 flagship current tech/aluminum and 2 older tech/plastic/cheaper.
As far as the resolution of the larger iPhone, most people really don' care whether it is HD, Quad HD or something else entirely. Only tech nerds whose opinions Apple ignores are interested in that debate. They simply want a larger display that matches or exceeds the current display. Were it not for the rather large chin and forehead of the iPhone I believe Apple would have gone larger before now. The circular home button and placement of the front facing camera makes it harder to make a 4.8" iPhone that is not quite a big longer than the Samsung SIII for example. The forehead at least is a relatively minor adjustment simply move the camera but I don't see any easy option for reducing the chin area occupied by the home button without incorporating it in to the display.
I for one will be putting my gold 5s on sale as soon as we get closer to a release and the rumors are far more substantial. I have been waiting for a 4.5" or larger iPhone for many years and find 4" extremely frustrating to use.