You are correct that course sizes matter to educational outcomes for a variety of reasons. If the groups are too small, it is possible that nobody has enough o

You are correct that course sizes matter to educational outcomes for a variety of reasons. If the groups are too small, it is possible that nobody has enough of what is needed for the class and everybody flounders, requiring intensive instructor led tutoring and learning. This brings us back to the 1:1 situation. If we are interested in how many students are required to support peer to peer learning, you might enjoy the following articles (popular and academic) about how Harvard physics professor, Dr. Eric Mazur has embraced and studied peer-to-peer instruction. This should provide a partial answer to your questions. 1) Eric's Harvard Profile https://www.physics.harvard.edu/people/facpages/mazur https://www.physics.harvard.edu/people/facpages/mazur) 2) A good story/video about the development of his peer-to-peer, active learning methods (https://www.youtube.com/watch?v=WwslBPj8GgI https://www.youtube.com/watch?v=WwslBPj8GgI). 3) One of Harvard's articles about his educational work. (https://www.harvardmagazine.com/2012/03/twilight-of-the-lecture https://www.harvardmagazine.com/2012/03/twilight-of-the-lect... 4) A list of Dr. Mazur's publications (https://academictree.org/physics/publications.php?pid=66072 https://academictree.org/physics/publications.php?pid=66072) 5) Eric's 2015 papers on peer learning a) 2015 - Dowd JE, Araujo I, Mazur E. Making sense of confusion: Relating performance, confidence, and self-efficacy to expressions of confusion in an introductory physics class. Physical Review Special Topics - Physics Education Research. 11. DOI: 10.1103/PhysRevSTPER.11.010107 b) 2015 - Miller K, Schell J, Ho A, Lukoff B, Mazur E. Response switching and self-efficacy in Peer Instruction classrooms Physical Review Special Topics - Physics Education Research. 11. DOI: 10.1103/PhysRevSTPER.11.010104 Googling for "ideal group size for peer to peer learning physics" brings up articles from Vanderbilt University and those mention groups of 2-4 or 5-7. As there are many different models of peer learning, this article does a nice job of discussing practice and the theory behind peer to peer instruction (https://www.ncbi.nlm.nih.gov/pubmed/29128431 https://www.ncbi.nlm.nih.gov/pubmed/29128431) My personal beliefs and experience is that people tend to like groups of 6 or fewer and the groups which speak the most among themselves range from 2-5 people. The limitations are that at least 1 person in the group must be advanced enough to help the others learn. Last, if we look at apprenticeship models of learning, we are back to the 1:1 ratio (expert : learner). However, any single student can also have multiple 1:1 "teachers" and apprenticeships/internships usually include a peer group which engages after the 1:1 instruction. We see this in music and sports with individual sessions with a variety of experts followed by group work or performances with varied sized groups. I hope you enjoy!