There are some different theories. Personally I think younger children benefit from a smaller ratio 15:1 and older children benefit from a slightly larger one, maybe 20:1.
If you want to stimulate good discussions then it's better to have a wider social group as possible.
@TheConstantGardener I also think that it's important to what each teacher like. I don't see why class rooms have to be equally broken up. If a teacher is better a teaching bigger groups than I don't see why they can't have a bigger class room or vice versa for the teacher that likes teaching a smaller class.
@Lostpoet In further education, class sizes are often very large and that's ok for lectures. Age has a lot to do with it and also the subject. It's difficult to generalise. The problem with varying class sizes is that it can difficult for the school management to get teacher recruitment to fit personal likes of the teachers. There has to be some kind of standard.