The only time SMU had any true attendance was during the Pony Excess days but there were more people in the stands who were being attracted to a fad than there were who were part of a fan base. In their own city the had to get in line behind Texas and A&M and many of the residents of the Dallas Metroplex had allegience to other schools (Oklahoma, Arkansas, TCU, Baylor).
If it weren't for the death penalty (and eliminating the fad followers) they would more often than not fill a stadium the size of the Rent (nor sure how large Ford is) but I believe their stabilized fan base is somewhere around 30k these days.