This link is interesting in that they take the television market, number of fans for the team from sources like Common Census fan map, and google search traffic--though the numbers were run back in 2010 when the last round of realignment was taking place.
http://thequad.blogs.nytimes.com/2011/09...haos/?_r=2
Here is how they rate the college football fan base size for candidates in Big 12 expansion this time (with conference alignment changes since these numbers were generated):
BYU 709,864
UConn 618,724 * Big East to American
USF 520,627 * Big East to American
UCF 506,679 * C-USA to American
Boise State 483,489
East Carolina 348,391 * C-USA to American
Cincinnati 322,757 * Big East to American
Arkansas State 312,058
SDSU 242,126
Memhis 235,656 * C-USA to American
Temple 226,483 * MAC to American
Air Force 213,946
Colorado State 199,506
New Mexico 171,154
SMU 163,671 * C-USA to American
Northern Illinois 162,687
UNLV 136,687
Rice 126,901
Tulane 101,701 * C-USA to American
Houston 100,253 * C-USA to American
It would be interesting to see if these numbers significantly changed over the past 5-6 years.