I've had several classes that had over a hundred people, and by the end of the semester there was usually only like 15 people in the class.
also if you're reasonably intelligent it shouldn't be all that hard to participate. I'm pretty shy but I usually managed to participate because I would know the answers/not care if i'm wrong.
also if you do any group projects/presentations all the stupid people will be too busy text messaging/talking to their friends/jacking off and looking smug to participate, so you'll end up doing all the work and get all the participation points
but who knows if our experiences will be similar!
if you're really shy just bring some rocks in to class and whenever the camera pans at you just throw them at the wall and then nobody will have to look at you!