I've run 24v24 on a 6"x4" table. It was fun and felt like We could have crammed another lance or two on the other side or maybe some vehicles. A big city board (like one using Dropzone Commander ruinscape) could probably soak up some miniatures - you could fit a bunch.
But, I think what it really comes down to is how many Mechs do you want each player to handle?
Then - how many players can fit on the board? It is uncomfortable for more than 6 players to fit around a 4"x6" game table - you start bumping into each other and someone invariably smells.
I am building a board now that will be 4x6 or 4x8" and will look like the Canyon Networks map from MechWarrior Online. I am hoping I can get a couple of Mech Battalions lost in there and have them mix it up. Would be really cool to try playing the blind play/ sensor rules in there.