Large. Strangely enough my PC (which is certainly not cutting edge) can handle huge just fine, but I actually find huge is too big and gives the AI loads of room to quickly expand into, making it a slog to even trim them, let alone take them down.
With the AI bonuses, they can expand like crazy and grab huge swaths of land in a way a human play simply can't do. This leads to them having many, many big cities, which in tern leads to them having a big tech lead and mountains of cash.
I find that unless I'm super-aggressive from the start and all through the game, the AIs become too powerful, too quickly and any later wars just turn into a nuke fest.