The ideal class size?

Attending a high school boarding school forum in Qindao China recently I was also given the chance to visit the top public school in the area. Many of the parents who were interested in bringing their childern to the US were also very concerned about the number of students in each class.
I watched as they looked at the bottom of every posted school's class size ratio. Parents outright avoided schools with a 20 to 1 ratio or higher and hovered around schools with a 8 to 1 or even 6 to 1 ratio. Noticing their patterns I thought about Malcolm Gladwell's latest book David and Goliath. Here Malcolm points out that a smaller class size is not necessarily better.
While it makes sense to see that too many students for one teacher will leave many students without individual help. What is interesting to note is how Gladwell was able to record the point of view of the teachers and their take on overall class dynamic. Teachers themselves would much rather teach a larger class than a class under 12 students. Too few students and it becomes even more difficult to generate interesting class discussions from many different perspectives. The ideal ratio seems to be 18 or 20 to 1 the same ratio being avoided at the boarding school fair in China.
Remember, class ratio's are not like golf scores were a lower number is better. Evaluate how your own child learns and determine if they need a smaller more personal class size that isn't too small to impede on their overall understanding of the world.