I think there is a market for 7, 10, and 12 tablets, but the sweet spot is probably 10. I would like to see Apple make several models, and then let the people choose the size they like. Then everyone is happy.
I don't understand why the other manufactures like Dell and Samsung are making smaller tablets (5 and 7). I think they would be better off copying the current iPad, put Android on it and sell it for a couple hundred less. They would still make money and would sell millions.