Yep, a study on class size in the new Best Evidence in Brief:

What difference do smaller class sizes, and more teachers, make in early childhood education (ECE)?

A meta-analysis by Jocelyn Bowne and colleagues, published in Educational Evaluation and Policy Analysis, attempts to find some answers. The analysis included evaluations of ECE programs in the U.S. between 1960 and 2007. The evaluations were either experimental studies, used a high-quality quasi-experimental design, or showed baseline equivalence of treatment and control participants. In total, 38 studies were included, all of which looked at children ages 3 to 5 years old attending an ECE center for 10 hours a week or more for at least 4 months. Child-teacher ratios ranged from 5:1 to 15:1, and class sizes from 11 to 25.

The findings were as follows:

• Above a child-teacher ratio of 7.5:1, changing the ratio had no effect on children’s cognitive and achievement outcomes. Below this, a reduction of the ratio by one child per teacher predicted an effect size of +0.22.
• For class sizes greater than 15, increasing the size of the class had little effect on children’s cognitive and achievement outcomes. Below this, one child fewer in the class size predicted an effect size of +0.10.

The authors caution that these findings are correlational, rather than causal, so changing class sizes or ratios, certainly at scale, may not lead to these results. However, they conclude that “very small and/or well-staffed classrooms might confer some small benefits for children’s cognitive and academic learning.”