Technical specifications are not the problem, at least I hope that won't be so, there's no reason it should be with the new game.
The problem, is that the game may not be designed at all to function well on huge maps. Civ5 seems likely to introduce huge amounts of micromanagement of armies and tiles in cities and so on, that just becomes painful on large scales. The AI might not understand how to cope at all, and many game mechanics - barbarians or buildings or wonders or whatever, may not scale to huge maps.
The clear answer is that civ5 will be designed to work on smaller maps, probably a little smaller than the average from civ4. We can hope it scales up well, but no guarantees...