Show up on a Wednesday with $5 for the bike and $5 for the class. Bring your own pedals if you have them. They have clips & straps for people who like to ride with street shoes, but if you're used to clipless, use what you are most comfortable with.
They've got a whole range of sizes, so you shouldn't have a problem finding one that fits.
http://www.obra.org/track/flyers/2009/wednesdays.html