Dave its a gr8 idea, I have participated with the free MS Online seminars before and they are a gr8 help as you get to ask questions there and then. Heres my Input?

Length: I say a lesson should be between 1.5 to 2.5 hours depending on topic, as some subjects take longer and then there is the students questions etc...
Lessons: I would agree with your suggestion 12 lessons over 4 weeks is about right
Communication: Offer of both of the in calls comminication would be best. If a student can't get a word in edgesways as someone is hogging the airwaves, then they can type the answer.
Class Size: I think you should not go over 12, it would become a total nightmare with more than that.
Subjects: Everything . I would like to see a nooby class teaching the UI, basic modeling and texturing and simple animation. An Intermediate classes for more complex topics, character design, character animation, environment creations etc... And finally an advanced class teaching dynamics, advanced animation techniques, nCloth etc...
Cost:Well free ones would be nice (IU based classes and some of the basic lessons for free (ya know samples)) But to attend a full cource at basic, intermediate or advanced levels then I would say anything from $400 to $800 (£200 to £400).

Really this raises a couple of questions for me:

1. How are you going to deal with the time differences?
2. Will any be available for download after the fact (at cost), for those of us who cannot afford to to shell out the money to attend the live course?
3.