I just got a medium timbuk2 bag, and as others have said it holds a lot. I can fit my lock, a set of clothes, some papers, and my ibook laptop into it. Grant it the laptop pushes it a bit to close it with the velcro when all that other stuff is in it.
I really like the medium, as it works well either full or more empty. It holds well on your back and doesn't slide forward easily. So while it may seem really big, I'd go for the medium if you need to carry much stuff.
"Only the meek get pinched, the bold survive"