I started writing a post about how I felt this was entirely a map design problem. Differently sized and structured maps could call for different sizes and makeups of teams. Period.
Well... maybe not so Period.
When you get right down to it, every map should endeavor to support the playstyles of every Legions player -- you don't want someone feeling like they can't contribute. No matter what you feel your "role" tends to be in a given game, you should be able to find your niche when you're playing on any given map, and fill that niche well. Take TF2: a solid TF2 clan will almost assuredly have most, if not all, of the classes in play at any given point in time. Likewise, in a WoW raid, you'll need two tanks, two to three healers, and five to six DPS players to ensure a boss kill, regardless of the fight. The tactics might be different, but the playstyles -- which are embodied in the classes being played -- stay consistent and are always provided for, regardless of the map, or the gametype, or the fight.
A good map in Legions (or any game, for that matter) should do the same thing: provide opportunities for each player on a (theoretically BALANCED) team to be able to contribute, regardless of the niche they're filling. That means the capper, the light offense, the chaser, the HoF, the heavy offense, the light defense, the (insert your role here), needs to be provided for. Maybe 7v7 is the magic number where EVERY person in EVERY role is contributing to the whole; maybe it's 10v10. And of course, there are going to be maps optimized for more players, and there are going to be maps geared towards fewer. But giving every player responsibility is critical in giving every player the fun-factor, and if you bump the number too high, your players can end up feeling a little on the useless side. Maybe there are too many players on defense and they're unnecessary; maybe the map's too crowded, and players that would have had specific goals to achieve instead find themselves overwhelmed in the chaos. But either way, it ends up knocking the fun factor down a couple notches for SOMEONE, even if it's not for everyone.
So, yeah, map size is definitely an important factor. But ensuring every player has a role is just as important. We should be careful when considering raising that number.
Unless we're just talking about deathmatch. In which case, bring on the hordes. =)
- Daph