The StarCon/StarFest cons in Denver were two-day cons well past when they first topped 3000 (in 1984, when DeForest Kelley was there). Last one I attended (over a decade ago) was a 2-day con with some early registration and activities on the Friday night (so call it 2 1/2 days, if you like) -- attendance was around 8000.

I think the first 3-day SF con I attended was Chicago TARDIS in 2003, and they only had an attendance of about 400.