Thank you, KS.  My average class size was anywhere from 30 - 36.  I taught 6 periods a day and 4 of those were teaching Expository Writing.  Except for sitting up quite late to grade and make comments on over 12 - 14 papers (including rought drafts) x 136 students per semester - I never had a problem with class sizes. The writing classes were taught to 10th and 11th graders and was required for graduation.  Of course, I had the rest of the periods for teaching English classes with an random Speech class or Drama class thrown in the mix.  I had no classroom assistants.  

In those days I read at one time that the right size was 18, but I believe that applied to the elementary grades especially 1 - 3.  

I've also done research and I know for a fact that one can make the research deliver the desired outcomes.  It's all in how the research is designed.