I did my doctoral work on cooperative learning at the University of Minnesota. One of the advantages of doing my work at that school was that I had access to the Johnson brothers, one of the foremost recognized thinking teams about cooperative learning.
One conversation we had was about group size. Often in corporate training the size of cooperative learning groups is driven by factors like the physical size of the tables (eight people can fit at a table, so we’ll have groups of eight). But Prof. Johnson’s assertions about the research were crisp: the positive effects from cooperative learning evaporate by the time group size reaches six. At six or more, it’s too easy for learners to disengage and let the rest of the group do the work.
I don’t always win, but I do always advocate for small group sizes!