I bought the medium originally for a Christmas gift, but it was bigger than expected.  I ended up keeping it for myself.  I am able to put my work computer, lunch, paperwork, all in this bag and have room to spare.  You could put a full change of clothes in here!  It does work fine also when riding, however I don't carry too many heavy things when I ride to work... secures nicely around your body.

Then bought the small.  Another great bag.  It has been used daily and still looks like new after a year.