Cathy, let me introduce you to A-kon...15-20k people
And these are small potatoes compared to Dragon Con & Comic Con.
Fencon is amazingly tiny for the rep it has.
DFWCon is an comfortably sized writers con, probably a bit smaller than RT, definately less authors, decidedly more than Fencon.
Aggie-con, btw, is one of the longest running cons. Small, intimate, but guests like Terry Pratchett, Harlan Ellison, etc.