If you live in Pennsylvania and are insured by a Pennsylvania insurance company you need only produce an insurance card to get attended to elsewhere.  Single-payer would work exactly the same way.  You would have a card issued by the State of Pennsylvania, and you would be fully covered when you traveled.

As far as economies of scale, yes bigger is better, but one State agency replacing dozens of bloated insurance company bureaucracies also represents economy of scale as well as the millions that would be saved by doing away with expensive marketing and advertising campaigns and outrageous upper management compensation packages.

No one is arguing against a national plan, but the fact remains that a national plan is unlikely to gain passage unless the states take the initiative.  Get it passed on the state level and Congress will follow sooner rather than later.