Battlefield 3 Senior Gameplay Designer, Alan Kertz has confirmed that the squad size in game's multiplayer mode is 4 on all platforms.
A user named "Demize" on Twitter asked Alan Kertz:
"4 squad memebers shown in HUD in the multiplayer tease trailer, is this the final amount or will more be able to join? Hoping for atleast 6"
To this Alan Kertz replied:
"Squad size is 4 on all platforms"
In Battlefield 2, the Squad size was 6. Battlefield 3 will launch on October 28, and beta for the game will begin in September.