have voted but afraid that answering rest of question would take a long time and I have work to complete before tmws lecturing. We presently have two groups of five, about 4 groups of four, several groups of three and a couple.
We did have a group of 6 but one had an eyesight problem develop that made the group difficult for him - so thats now a group of five.
Personally I would have said real variation in each group - they just need to establish relationship to each other - if there is a bully and one being bullied I have found it better to subtract the bullied one (seems counter-intuitive buts its worked for us every time its happened). If yo subtract the bully then the whole order has to be re-established. Sbtract the bullied one and the group settles down.