This is as much a request for information as anything else, and I imagine that it is information that other academics want too!
Most of the courses my department teaches fall into one of four categories. There are large and small courses, and there are courses aimed at majors and courses not aimed at majors. Plenty of small courses are not aimed at majors; most large courses are not aimed at majors (the exception, sort of, is 101, which is aimed, partly at least, at attracting majors). In our context, large means 80-100 (with, occasionally, 160); small means 15-30.
I suspect, as do most of my colleagues, that more learning-per-student occurs in the small than in the large classes. This is far from certain, because we lack high quality measures of learning. We also assume that some teachers are better than others in large courses, and some are better than others in small courses, but we don’t know a great deal about who, because…. we lack high quality measures of learning.
We are not going to engage in a wholesale reform of our curriculum (that’s a prediction, not an insistence). Like most departments, though, we have some latitude in deciding how large our courses will be. The key choice that we can make is in how big to make the big courses, and how small to make the small courses. So, for example, we could keep our large courses as low as 80, and pay the cost in terms of allowing caps on our smaller courses to rise to (say) 30. [Of course we could decide to teach all, or nearly all, of our classes, as mid-sized, and if you could find me research that convinced me that was optimal I would share it with colleagues and try to persuade them, but I am putting that possibility aside to simplify things]. Because we lack high quality measures of learning, and because sample sizes are anyway small, we can’t make those decisions on the basis of what we know about our own situation, so it would be handy to have research that could tell us something useful.
But the research I have seen doesn’t tell me anything useful.