NYTimes article on metrics used to determine the size of fanbases nationally and locally. Interesting read form 9/2011, with some cool maps to back up their data.

BYU comes in at #43 nationally in terms of overall fanbase size. Utah is #67, with one-half the overall fan base of BYU in total numbers. For reference, Louisville is #73, Cinci is #70, Clemson is #10 and FSU is #38. BYU is actually in great company nationally.

"Brigham Young, as we mentioned, would be an attractive-enough addition for a conference like the Big 12. With about 700,000 fans, it ranks behind 42 automatic-qualifier schools but ahead of 27 of them."

http://thequad.blogs.nytimes.com/2011/09/19/the-geography-of-college-football-fans-and-realignment-chaos/

MAP:

http://www.commoncensus.org/sports_hotspot.php?radioDiameter=200&map.x=147&map.y=127&sport=5