I need a new bag to carry stuff (mostly books, U-lock, laptop) back and forth from school on my bike, and the weekly grocery run for a single person. I've pretty much narrowed the bag down to the BW Super Pro, which I've heard are fantastic bags, but I'm not sure about size. I tend to think a medium would work well, but I'm a smaller guy at 5'7", ~130 lbs. Not sure if a medium would be too big on me. I know there are measurements on the site, but it's still not always clear how measurements translate to how the bag sits on one's frame.

Also, I would like to use the bag to walk around sometimes too. I wouldn't store much in it in such cases. Perhaps a small GPS unit, sweater, food, beer or two, etc. Would a medium size bag be unwieldy for me so that a small would be better? Does anyone who have experience with these sizes or are on the shorter end of the scale care to comment?