Believe it or not, the research findings on this are not that strong. I should see if I can look them up. I know it sounds unbelievable and my instincts tell me otherwise, too. However, my DD got a better education in a class of 18 than in a class of 11.
ETA: OKay, so there is one large study (Project STAR) that showed some decent effects for grades K-3. Other than that, the effects are quite mixed. Florida implemented a class size limit at great expense. Thus far there have been no clear positive effects:
http://www.hks.harvard.edu/news-events/news/press-releases/pr-pepg-research-may10
