I very briefly had a Nexus 6 (6, NOT 6p), which had a 6" screen.......and I -----LOVED----- the size of the screen!
Not as bright and dazzling as the Samsungs, including even my older Note 3 that it replaced. And the colors weren't as natural or the white balance as nice, bright, and white as the iPhone6 I have now. But despite all that, damn it all anyway, in the final analysis, the Nexus 6 has been my favorite screen on any phone I've owned so far! Part of that is due to the crispness of the QHD resolution...but even more, because of THE SIZE!!!!
So, yes! I would love a 6" iPhone. I'm not sure how much bigger than that I'd want to go as "bigger is better" is definitely a finite proposition....but I certainly think the current 5.5" is quite a bit lower than that hypothetical metaphorical ceiling.
Now, you can file this next bit under "so unfeasible that there's no way it'd EVER happen" pipe-dreaming, but what I'd personally love to see is a whole spread of sizes to appeal to everyone's taste. Let's say every size denomination in 0.5" increments between the iPhone4 small 3.5" all the way up to a 6" Nexy 6 class mammoth, or maybe even a 6.5" Note Mega caliber of ridiculousness!
I understand exactly why that'd never work. It'd fragment the market way too much. Each model would cost too much to make, and then sell too little to recoup. Plus then you'd have to make sure everything looks right on 6 or even 7 different screen sizes instead of just the two they have now. And it'd run the risk of confusing and alienating the consumers especially with as cluttered as it'd make the sales floor in the Apple Stores.
So, again, I'm not actually suggesting they do this. Please don't think that. I'm simply saying that I personally would LOVE this scenario!!!!!! Love it! Love it! Love it!