iPhone 5 -> Nexus 5/iPhone 5S -> iPhone 6+.
I think a display size at around 5.3" is ideal. The Nexus 5 was a great size and felt great in the hand, but the larger display is appreciated. The 6+ might be a tiny bit too big, but I can live with it.
The iPhone 5 I have around feels ridiculously small now. I can't even type on it anymore, it's like a toy or something. The camera is also terrible compared to the one on the 6+.
But I have to say, the iPhone 5 is gorgeous in a way that the 6 really isn't. The iPhone 6 design looks OK, and it feels good in the hand, but it isn't especially attractive.
Battery life is great too, although anandtech shows you can totally kill it in 3.5 hours if you play 3d games, which is definitely true. That said, if you don't play games on it it will last for well over a day.