Cant Change Game Size in Multiplayer

I can’t click the game size button to increase the number of players in matchmaking so I’m restricted to only 2v2’s. I don’t know if I’m incompetent or if the game is just bugged.

I assume you’re referring to the PC flight of Reach?
If so I’m fairly certain that’s intentional; they’re running scheduled sessions of different game modes and sizes, presumably to prevent spreading the limited group of Insiders so thin that they can’t find a game.
Your flight extension email should include the scheduled times and the modes featured during them.