I know some may wish to see epic battles with scores or even hundreds of water warriors laying utter soakage on the field of battle. However, smaller teams make it simpler both to find enough members to fill as well as to practice with. If a local group finds it has scores of interested water warriors available, these individuals can be divided into squads to battle and hone their water warfare skills with each other, switching team mates between games if desired to see how different groups function together.
Of course, need at least two people to have a team. For a tournament, I'd imagine having different levels of games based on team-size. While it'd be nice to also have variable-size teams taking each other on, I'm still of the belief that it is best for games to consist of equally-sized teams such that there is no question of an unfair advantage and/or disadvantage had by one team simply by number of members. Please don't bring up the theoretical situation of a few elite warriors able to take on hundreds of n00bs; while it may be true in a given situation, for comparison-sake, it is better simply to hold some variables like team size constant. 20 veteran water warriors against 3 novices would make for a rout. At least if team sizes were even could teams then be judged more by skill.
Also, for sake of argument, we'll leave out teams-of-two. I like the idea of pairs as a team for battles, but that option, to me, is already a given.
What I'm more curious about is whether the membership can come to a consensus on how large an "official" team should be. Baseball has 9, Basketball has 5, Ice Hockey has 6, Volleyball has 6, football uses 11... I suppose overall team size for a water warfare team can be larger, per se, but the number of active members during a game should be fixed, IMO.
