Could you point to this data?

This is what I normally point to although more because they give a lot of good references and are online than because of their own research. Unfortunately, there are a lot of complicating factors here. In particular, suburban schools with generally white students from high income backgrounds are more likely to be in small class room environments, and school districts which try to keep class room sizes small are also very likely doing other things to also improve the learning environment. I can't point to anyone who has corrected enough for all of those factors for my general satisfaction. I'd describe the data summarized in that link as strong but not conclusive.