IMO: I think that you should look for a phone that matches your play style. Obviously a larger phone would help you see the target of you're trying to shoot from afar. Small phones would be lighter and less visible, good for run and gun maybe, or even medics.
As for battery life, just make sure you have a couple external battery cases that you can use (one charging while using another).
Think "paintball" style Players choose markers and accessories based on their play style/position. In this case, you should choose your phone and accessories based on what you wanna do in the game
I wonder how people would look playing with iPads or tablets