First of all there is no such thing.
Second, a friend asked me the question “What is the idea group size for collaborative process?” and in trying to answert the question I emailed him the following (please note that this is all off the top of my head, and in practice I usually go with intuition, relying more on patterns than rules):
Innovation generally starts with individuals, so I like to build time into to processes for people to just be quiet and think for a bit. Small groups can help refine and test good ideas, and large groups can help propagate ideas and connect them to larger patterns. In small group work, in general, working with an odd number is helpful because it creates an instability that keeps the group moving. If you want solidity, you work with even numbers. So it goes like something this:
1 = innovation, idea generation, inspiration and commitment
2 = Pairs are good for long and exploratory conversations, interviews, and partnering
3 = Good number for a small team to rapidly prototype a new idea
4 = A good number for a deep exploration. You benefit from having two pairs together, and from having a little more diversity in the group than in two.
5 = good number for a design team; there is always an instability in a group of five and good diversity, but the group is not so large that people get left out.
6 = Good for noticing patterns, and summing up. A group of six can be entered from three pairs coming together as well, allowing for insights gathered in pairs to be rolled up.
7 = At this scale we are losing the intimacy we need for conversation, and so generally I will work a group of seven into 3 and 4 if we need to break up.
8 = is too big. And it is no coincidence that big conferences are boring, because most hotels have tables that can accomodate 8, 9, or 10 people which is too many for real conversation. At these scales, people start to be able to dominate and introverts dry right up.
It is a good practice to use a huge group (like in the dozens or hundreds) to source the diversity that is needed for good dynamic small groups, and then to find ways to propagate ideas from the very small to the very large.