David,
The MOOC model is also becoming popular; however, not much research has been done on the effectiveness of the MOOC model. I think your final two sentences about engagement are very important. Students that typically feel more engaged and connected in their learning are more likely to stay involved in online learning. We lose students when they feel disconnected from their learning, instructors, etc. The 15-20 for the typical class is what most people identify as ideal.
Herbert Brown III
