What is the optimum class size?

How important is class size and how does it relate to learning opportunities? I started thinking about this after reading an interesting post on Patrick Watson's Montrose42 blog. Watson summarises some theories put forward by Malcom Gladwell, who suggests that a class size of 18-24 learners is considered to be an ideal number. Obviously, very large class sizes may lead to reduced learning opportunities. However, teachers also reported that very small class sizes can impact learning negatively, for example by a disruptive student having a proportionally greater effect on the classroom dynamics. 

I have taught classes ranging  in size from ten to fifty learners. Clearly, fifty 14-year olds in a classroom is a far from ideal learning environment! However, having just ten learners does present some issues of its own, for example small group presentations don't generate the same level of interactive engagement during the question and answer session following the presentations as a similar Q & A with a class of twenty. I have noticed that the energy level of a very small class is frequently lower than in a class of 25 learners. Obviously this is an anecdotal and not particularly objective finding but it is certainly a phenomenon that I and my colleagues I have discussed this with have experienced. 

The upside of a very small class is the opportunity for a teacher to spend more time with individual students. There is also greater opportunity to explore tangents that may arise from learners' questions. However, Gladwell notes that better learning outcomes are often not achieved in small classes because teachers don't adapt their teaching to meet the needs of a smaller class - they simply work less! I find this a bit surprising, because in my experience a lesson plan that is suitable for 25 learners won't necessarily work with a class of ten learners without at least some modification. 

I would be interested in hearing other teachers' opinions of what they think is the ideal class size.