Back in the late 70s Gene Glass did a meta-analysis of the research on the relation between class size and achievement. I vaguely recall the magic number was around 18, that is, there was not very much effect until you got down around 18 (don't quote this). This was disappointing because such a reduction is not considered economically feasible, and not much was done after his study was published.
There might be something in the Handbook of Research on TEaching.
As pointed out by Fennell in this forum we should look at teacher satisfaction as an important variable. There is research that says one of the best ways to improve productivity is to improve working conditions. Isn't that what Saturn(autos) is about?
Now how do we convince the public that they should pay for what most pundits consider a luxury?