Before revision:  An optimal size is defined to be the size which maximizes the probability of no default of the group 
After revision:  An optimal size is defined to be the size which maximizes the probability of no default of the group . To do this, one has to create a model with interacting forces, and to make precise the hypotheses of the model. (In previous work we have addressed the issue of what is a fair rate of interest to charge for such a loan.) We show that the original choice of Muhammad Yunus, of a group size of five people, is, under the right (and, we believe, reasonable) hypotheses, either close to optimal, or even at times exactly optimal, i.e., the optimal group size is indeed five people.