TTAC in Toronto has a very small area (I can't estimate distances to save my life, so I don't know the sqftage), that fits 8-15 players pretty well. It's VERY close quarters, the games are fast and hot, and you rarely find clear firing lanes from one side to the other (the "walls" are just sets of three pieces of particle board ziptied together, so it gets setup differently every game night).
On the other hand, Toronto also has the Ultimate Airsoft field, which is significantly larger, and has a lot more open space. Rather than feeling like you're fighting inside a single building or two, there are buildings within the field itself, that you can move between/through/around. there's also a "hurt locker" area which is a very tight set of corridors, similar to TTAC, that you can move through if you don't want to traverse through the open.
So TTAC has some intimate and very fun games in a tiny space, while UA has much more open space. Unfortunately, even with all that extra space, I find that UA is setup in such a way that people still tend to bunch up at cover when there's, say, 10 players on each side.
So while the field size is a good starting point, the setup of field really dictates how many players you can fit on it.
I know that isn't really what you asked, but it's the internet, and I like to type things that don't matter.