Hi Rachel. I teach only children ages 4 through 11. Depending upon what you're teaching that particular day, class size doesn't matter.
My 4 year old class is usually six little boys and that is probably optimum for their age and attention span - the class is a half hour, which is more than enough for them.
For the older kids, if we're doing techniques or forms, there can be any number of kids, as long as they have a partner or group (3 usually) to work with. If we are doing drills, the same applies. If there are lots of kids attending class we try to keep them occupied but -- kids being kids -- they like to fool around, and that is not limited to the boys, I'm sorry to say! I like ending classes where they've done well with relay drills, which they LOVE. Also tires them out for their parents:boing1: