You can always hold your phone up to the screen and determine if a new one is going to be a bit too large...

http://www.piliapp.com/actual-size/

its not perfect, you cannot simulate wrapping your hands around the device but you can actually hold an existing device up and get a good idea of what it will be like

Sure, and on screen the S5 might not look too big, but in my hands it felt ungainly and awkward for anything longer than a few minutes. The point I was making was that the 6+ might be a better tradeoff with less battery life but a slimmer profile to 'fit' better in hand.

On the other hand, the first thing I did when I got my note 3 is buy a case so it wasn't so blasted thin. I think the fact that there is barely any side edge on these phones makes them difficult to hold onto.