The Middle Size League (MSL) is an official RoboCup league. Two teams with 5-6 robots than can have up to 80 cm height, 50 cm in diameter and 40 Kg of weight, challenge each other in a football field. This field is similar to the human one which has 11 players, but with a smaller size (18m x 12m).
Learn more about the Rules.
Middle Size League: http://wiki.robocup.org/wiki/Middle_Size_League