While we're chatting so pleasantly, what size board were these rules intended for?
The simple answer is, whatever size you've got.
The point of the rules being so simple, is to allow literally hundreds, and hundreds of zombies on the table at one time.
Whoever runs your game should set a realistic objective, or balance of forces, depending upon what you've got available....basically play the rules - if you get slaughtered in the first 10 minutes, rebalance the sides

To give you an idea, my table is 4'x10', and we typically have 400 zombies on table, with 3-6 players. Depending upon the scenario, each player might have 1,2, or 3 figures each, plus maybe access to heavier weapons, (such as a LMG), or a vehicle.
Experiment with different size forces, and play the rules half a dozen times before being tempted to tinker with the rules - they are the way they are, after being used inplenty of games.
