What is the "optimal" group size? The "right" number depends on purpose, but it also depends, to some extent, on human capabilities.
Christopher Allen wrote a series of posts on group size. The sizes he proposes center around the Rule of Seven and the Dunbar Number. For example, he suggests that working groups should be between 7-13 people. (He refers to this upper limit as the "Judas Number.")
The number three has some importance in network theory.
Breakouts or Whole Group?
One process question that always comes up is whether or not to break out into smaller groups or whether to keep a whole group together for discussion. Many things factor into this decision. First, it's important to note that every breakout should be followed by a whole group report-out, so synthesis / cross-fertilization happens. Second, it's also important to realize that -- despite our greatest hopes -- it's not possible to include large groups on every aspect of a conversation.
When considering whether to breakout into groups, and if so, the size of each group, you can use a simple equation as a guide:
- Time / Group Size = Individual Time to Talk
In other words, if you have an hour and a group of 15 people, each person will have 4 minutes to talk if everyone takes an equal amount of time (which almost certainly won't happen. If you take that same group and break them out into three groups of 5 for 30 minutes, then each person will have at least 6 minutes to talk, and you still have 30 minutes for whole-group synthesis.