I had the 6 Plus for about two and a half days, during which I tried my hardest to get used to the large size. My complaint is the same as yours, such that I can't use it comfortably single handed. I'm right handed so my biggest problem was hitting the numbers key on the lower left side. Not only does this interfere with typing numbers but also displaying the punctuation keys, including the question mark. Also, I was constantly shifting the phone in my hand to reach whatever and I imagine it was only a matter of time before I dropped it.
Consequently, I exchanged it today for the 6 at an Apple Store. Good thing I wasn't trying to do the reverse because who knows when the Pluses will be readily available in stock. Since it's an exchange, you can not reserve one online which will make getting one particularly difficult. The 6 I wanted was in stock but it was a 3 hour wait for a sales person.
I'll miss that screen though; I was really awe struck by it. The 6 is better than the 5s, but the Plus was better, which is to be expected from the stats. I just didn't think I'd notice the difference as much as I did. I'm really hoping the new iPads offer a similar experience.
I've now had the 6 for a few hours and I'm pretty certain I made the right decision. Every time I pick it up, it feels so much more natural. Good luck with your decision.