If you really want to use an iPhone 6 or 6 Plus with one hand, just double-tap (not press, tap) the home button and the software adjusts the display size for you.
Unfortunately, I found even the 4" too large for completely comfortable one-handed operation. Shame I'll never see another 3.5" iPhone option.
I expect a Plus has an even worse hold and I wouldn't be able to use it 1-handed at all.
This seems to make sense, intuitively, as well: I'm applying way more pressure to the bottom of the phone with a firm press of the home button than I do when I lightly tap it, even if I have to tap it twice.