I hear that groups 10-15 are ideal. If your group is large, split them up. Arrange the group in a circle (shoulder to shoulder). Have them raise their right arms and grab the hand of a different person (they cannot choose someone next to them). Next, have everyone raise their left arm and grab the arm of another person (they cannot choose someone next to them nor can they choose the arm of the same person). The goal of the game is to untangle themselves in the form of a circle without breaking hands.
This is a great way to get students interacting and working together. If the group fails, have them start over.