I don't want to sound cynical here, but I understand the financial reality of choosing between doing six tandems or teaching a canopy class. Regarding class size, at a Cessna dz a class of four would be ideal. I limited it to six. The students need a little traffic to sort out, but not too much. I believe that four or five is the optimum class size.
Larger groups are also unwieldy in time.
It takes everyone longer to pack, get manifested, sit through the briefings, etc. For optimum efficiency, stick to four or five and have then all exit on the same pass. This keeps the class moving as a group. If drop zone logistics don't permit moving as a group you will have to be creative, but it really helps to debrief as a group since so many common errors will be shared by more than one participant. As soon as they land, they should get packed while you review the video in private and note what you will cover in the debrief.