Originally Posted by ch0mb0
I have the large (23 inch) bag, and it easily accomodates a change of clothes and still has room for plenty more. You can really load them up. Sits on the back pretty good, though I haven't done a 20 mile ride with it on. Works well so far.
If you get one, make sure to get yourself the pad for the strap too.
I have a medium and it fits a change of clothes for me. I thought about getting the pad and a holster for my cellphone and mp3 player. Does the pad really help?