It seems like Apple doesn't plan to make the iPhone any wider, so the only option is taller. :/
4.3" in 2017, while everyone is at 6". Lol.
While many see screen size increases as improvements, I think the point Apple is making is that the screen size is a preference in user-experience. It's not that Apple lacks the technical know-how to employ bigger screens. They are concerned with one-handed operation. Specifically, the use of the thumb to access most of the screen. I think Apple will stick to this. I believe, in the current form factor, that if they can reduce the bezel, that they can fit a 4.3"- 4.5" screen without adding too much to the width and height. The idea that the phone will just get longer and longer is silly. Apple stretched the device to allow for better widescreen use (see the updated Netflix app over the old app). That's all. I don't think they have any intention of having a screen size arms race with its competitors.
I believe Apple will stick closely to the current width, without need to adjust the length.
There is the law of diminishing returns as it relates to screen size. Bigger is not better, just bigger. Truthfully, I wouldn't be surprised if the form factor of the Samsung S-series stayed static, where it is now (maybe with some bezel tweaking like I described above, for the iPhone). I think the reason they released the Galaxy Note as a hybrid device to sort of bridge the gap between tablets and phones. Even they must believe that they are already at the upper reaches of what is a comfortable mobile phone form factor. It wouldn't surprise me in the least, if Samsung introduced a smaller form factor of it's flagship device to compete with the iPhone, for customers who prefer a smaller form factor for their phones. They won't keep upping the screen size for each new device. Seriously, is a 5.8" screen better that a 4.8" screen for a phone? That's like a little over an inch shorter than a 7" tablet.
I have man-hands so I can handle a little more real estate on the phone, but my girl has lady hands and big phones like the GSIII are problematic for her and single-hand usage (we tested the GSIII and it is not possible for her to grasp the phone with her finger tips on the opposite side of the phone, with the other side secured in the palm, allowing for free use of the thumb to access most points on the screen. She had to flatten her fingers behind the phone, with the other side of the phone rested in the joints between her fingers and her palm to get some reach across the screen. Of course, this is more precarious a grip and less than ideal for phone use. She eventually had to use two hands on it, like a little tablet. So I understand the point, well.
For Apple, since they only make a single sized phone, they have to keep it sized for all people, big-handed and small-handed. Nowadays, children have phones (Which I think it is a good idea. I remember when I was a kid, an emergency away from home required having pocket change and a payphone, if you could find a functional one.). It wouldn't surprise me if Apple introduced an additional, larger model for those who want more screen space, I don't believe that it will be any bigger than 4.5" to 4.8", ever, but I doubt Apple will ever stop producing the existing form factor, as it is pretty perfect in size and portability, for a mobile phone...for most people. That being said, I have some giant friends for whom the phone is a little too small for. I have a cousin who is 6'11" and while comical to see him use his iPhone (truthfully, any friggin' phone), I feel his pain.
My $0.02.
