I've heard that it's huge, but I'm not going to believe attendance numbers from a commercial event.
What sort of people attend? Are do there seem to be a significant number of compulsive readers? I need customers who are attracted by a table covered with text, not put off by it. Are there book dealers at Chiller Theatre? Large ones who keep coming back?
Are the a lot of the attendees geeks and/or fannish? Do they seem to have a fannish sense of humor? (I did quite well at a Unitarian General Assembly-- I didn't know it was quite such a fannish religion.)