I'll take a stab at some number theory here for size of groups. As a school teacher in a very small school district I've had the experience of having class sizes ranging from 5 to 34. As a conference presenter I've worked with groups of up to 100. There are some very specific things to take into account at various sizes.
2-3 Accountability Groups
I use this term because groups of this size are fantastic at holding others to deadlines, getting people to open up about their own faults and issues, and are a great tool for growth. They are often times Mentor/Mentee relationships or a meeting of colleagues trying to work on a very particular project. If you're trying to get work done, groups this size are ideal
4-7 Dinner Party
Groups that are bigger than a small gathering don't have the intimacy for people to open up to the same level unless they have been taking place for quite a long time. At most a situation like this can end up breaking up into a couple of conversations. Most classroom techniques have a lot of trouble at this size. A single speaker with 4-5 people listening is awkward. Breaking up into roleplays is also odd because either you have everyone doing them while the rest of the group watches, or you have two or three roleplays going on at once. This size group is best for brainstorming and having open discussion of a topic.
At this size you can easily have people break up into small groups for individual work or roleplay without things getting weird. Speaking to a group this size as a classroom doesn't work great but is feasible. Having something for people to do in groups of this size is necessary. At the very least taking detailed notes, writing down answers to questions, or having some form of team work helps a groups this size move forward. Discussions at this size tend to be dominated by a few key people unless you have some sort of system in place that gives everyone the chance to speak. Wraparounds, where everyone shares something in turn, works great at this size
When you get much above twelve it can be difficult to keep everyone actively participating in whatever is taking place. The classroom structure often times includes breaking people into small groups in order to achieve meaningful discussion. Question and answer works at this size but don't mistake that kind of interaction for true discussion. Some form of presentation followed by individual or small group work is generally needed to make the best use of this size. Sharing work from the group works to some degree but is difficult to make meaningful without a group meeting more consistently.
40+ Conference Presentations
Once you break the 40 line you have no ability to monitor what people are doing. You can't really stop the show to get feedback and make adjustments. There's likely to be too many questions for you to ever answer them all. This is where conference presentations differ from the other setting sizes. You're goal is to inform and inspire your audience to take certain steps in the future. You're focused purely on the takeaway with full knowledge that nothing will be completed during the presentation. Audience participation is still important, but audience input generally won't shift the direction of anything. You ask questions that you already know the answer to in order to keep the audience involved, not to gain any specific information.