5m is prob good, 2m maybe better, but of course this is not really applicable to bigger cities which are already much bigger than this.

10m is still a bit too big, you still get several regions with different needs and center-edge disconnects. I'm sure Slovaks would say the same thing, "Bratislava vs the east" etc.

Who knows if it's a good idea anywas? Would it create some very poor countries next to a rich ones just because of some geographic features? Honestly, I don't know even if I like the general idea.