Hopefully next generation there will be the 6s Mini and the 6s Plus.
Most people I know who are upgrading their phones are picking either a 5s or a 6+.
Only one person I know went with the 6, and only because their desire for having the latest phone won over their size preference. (though they seem happy with it now)
The 3.5" screen was perfect for me, but I'd settle for 4".
I have larger than average hands but I find even the 6 too big.
Maybe I just have short thumbs, or other people are more comfortable with a less secure grip on the device when using it one-handed.
That might explain how everyone else I know with an iPhone has cracked at least one of them, if not several.
I primarily use it as a phone. It's for quick messages, emails, taking photos, listening to music/podcasts. It's rare that I use the internet on it for much more than a quick Wikipedia/IMDB lookup.
I don't use it as a two-handed entertainment device for gaming, watching TV/movies, spending all day on Facebook, or trying to read books on it, so I have no need for a larger screen.
The only time my current 4s feels small to me is when apps don't let you set the font size. The font on iOS is huge by default and wastes so much of the screen. Even the smallest option in iOS 8 is larger than I'd like.
So why don't I stick with the 4s, or buy a 5s? Well for one thing, I need more storage than I have on the 4s (it's not 64GB) and even the 5s is larger than I'd prefer.
I'd like something newer than the 4s because the camera on it is terrible, and it's missing features like TouchID, ApplePay, and the motion co-processor.
A new 5s is almost the same price as the 6 here, so it's bad value for money, and is now only available in 16/32GB models. I want a 128GB or larger phone. (ideally 256+)
And the camera on the 5s is not very good. It produces noisy images and is slow to focus. If I'm paying that much for an upgrade, I want phase-detect autofocus, and ideally it would have image stabilization too. The 5s body is thick enough for that without having the camera stick out.