When I switched from the iPhone 4s to the iPhone 5c, the display size was somewhat altered with the 3.5 to 4.0 inch display change. It's not as comfortable as I would hope. I was reasonably happy with the smaller display. Putting another row of icons on the display wasn't my priority.
We're expected to believe that the iPhone 6 will have two sizes, such as 4.7 inch for a typical model, and a 5.5 or 5.7 inch phablet-sized model.
I know that some people have larger hands and yes, typing with thumbs can be a problem. It's especially a problem with Android-based devices, as I've found with the LG Optimus S, 2012 Google Nexus 7, and 2013 Nexus 7. You'd think on a high resolution, larger-display device that the keyboard would be spacious, but it is still a bit cramped, though better with 4.4.x than with 4.1.x.
Apparently, displays are becoming less expensive somehow, as we reach for intense densities. Otherwise, how can they afford to put such displays in smaller hand-held devices such as phones? Phone carriers aren't going to accept more subsidies/discounts degrading their profits. Besides, in some parts of the world, people have no subsidies at all. Often, their contract (or lack-of-contract) deals are better than those in the U.S.A. or Canada.
Of course, with a larger display, efficient or otherwise, more battery power will be required. Since the case is bigger to hold the display, that automatically provides some thin real estate in which to add batteries.
If the display requires more power than the current displays, of course, it will need more battery capacity just to stay even with the current phones. Hopefully, they've found some magic beans, errr, magic screens.
My first color display-using phone required too much recharging. So did my first 3G phone, and thankfully, the iPhone 5c chipset for LTE seems a bit more thrifty than the previous chipset in the iPhone 4s, although in 3G mode, it seems to quickly drain because of poor connectivity, tower or phone-related.
When I see reviews about an airport, they often mention electrical outlets. Isn't it sad that we're so tied to hand-held devices that we need to find outlets?
I'm not sure of the actual sizes, but it seems obvious that Apple will put together some bigger devices, perhaps, as they did with the MacBook Pro--three sizes, to compete with the larger Android-based devices. I'm just not sure I'll be ready for them.
Update 2014.05.08: I saw something the other day suggesting that tablet sales have dropped a lot, in favor of phablets. If that's the case, then, Apple should be targeting the space with both 4.x and 5.x devices. I'm still becoming acclimated to the 4 inch display. I think it is good but a bit on the large size. Other hands will disagree. As long as the pixel density is similarly tight, I will be fine with a larger phone when I switch. I can't imagine having a phone with a display larger than 4.5 inches but things may happen and I'll have to go with the crowd.
Update 2014.12.17: 4.7 and 5.5 inches for these two. I got an iPhone 6 at the smaller of the two sizes and it's not quite comfortable. I moved to the 4 inch iPhone 5c fairly easily from the iPhone 4s, and to that from the second generation iPod touch.
As far as I'm concerned, a phablet wouldn't work for me, nor would a tablet that of 5.5 inches. It's both too big and not big enough.