My largest experience with online FPS games is Day of Defeat and more recently Battlefield 2, DoD max size is 32 and BF2 has huge maps for 64 players.
If there was a large enough map with some serious choke points (I like choke points) I would definitely prefer 32 players, but the current maps fit pretty good with 24 people.
here is why I really like choke points, If you are playing with a pick up group, just joining an online game they are easy to defend is my first thought, my second thought though is that it forces you to play with your team mates, if you don't work together using Uber or have a balanced team you are not going to break through