I know that some studies have said that three or four students are the best number for student small groups in the classroom. However, I’ve always found that — at least for ninth-graders — working in pairs works best (though sometimes near the end of the school year I’ll have them do a project in three or four after they’ve matured a bit and gained more experience in pair work).
A new study now seems like it might reinforce my opinion. 2 people can learn to cooperate intuitively, but larger groups need to communicate is the title of an article reporting on research that two people can work more easily intuitively.
You might also be interested in some past posts about student small groups:
I’m adding links to these posts to The Best Sites For Cooperative Learning Ideas.