Is small, medium, and large really too complicated for a customer to understand? In particular if they're all identical otherwise (aside from battery remaining proportional); it will be an easy decision: pick the phone in the size you want. I can actually imagine Jobs releasing three sizes, but parity is key to promote simplicity.

I'd argue naming convention is important. They should just call it the iPhone [whatever] S, M, and L respectively. None of this iPhone 5se or 5sx which have been rumoured. Too confusing.