Honestly, the smaller the better. I posted the other day several 1v1 games, but have still had no luck. It's possible people have given up looking for them, but you realize if you make a 6 player game and even 1 player decides they don't want to play anymore, the rest of the players will be stuck waiting until eternity for the player returning. The more players you add, the bigger the risk. As host, I think you can kick players, but I don't know what that will do within the game.
Understand that this will be a slow process. In a cloud game, you forced to exit after each turn while the game is saved and then go through your unique civ's loading screen after every turn. It's the kind of game that will be played intermittently from day to day. Therefore, I'd make numerous games available that you're willing to play. Don't worry about playing them one at a time. Make a 1v1 available for me to join with you or join one of mine. Make your 6 player game, too, just in case, though I think you'll never bring it off. Make others, and just hope enough players are willing to join. It's not like you'll have so many players playing simultaneously that you have time to play a certain game. You're going to be waiting around a lot.