Fan interest is quickly effected by moving popular players both Localy and Nationally. I have seen a significant direct effect from "Interest" to attendance.
If you find out anything specific post it here. I put up a "
Fan Loyalty" thread on 12-3-09 and didn't get much accurate information. I have asked the 5-6 Beta testers I've interacted with and haven't gotten much from them.
Market size I do not know anything about.
I also need to disclaim that my views are from one team over 9 seasons in one Online League. This is in no way a systematic study of effect.
Good luck,
LP