In an unpublished Continuous Quality Improvement study of the effect of class size and mode of instruction at the University of Missouri at Columbia, Trish Roberts-Miller concludes that

Class size did make a big difference if people used the environmental mode. In other words, we more or less confirmed Hillocks' research saying that presentational mode is not an effective way to teach fyc [first-year composition]. We also confirmed other research that says that reducing class size will not break people out of presentational mode. So, I'd summarize by saying that reducing class size does make a difference--but not as big a difference as changing teaching mode. 

In the Seton Hall writing program we teach predominantly in the environmental mode. Instructors are expected to understand the strategies and processes necessary to teach how to write an essay and then communicate them, not primarily through lecture, but through problem-setting and solving, in-class writing, small-group discussion, peer editing. These ways of teaching require small class sizes.

Roberts-Miller, Trish.  "Class Size Vs. Performance."  Personal e-mail (24 Nov. 1999).