well...if you get the chrome i would get the largest one. There's always room for options for "smart packing" then. If you get an intermediate sized one, you may run out of room. The problem with books is that they are very dense. Weight is harder to handle than size with messenger bags. i'd rather have mine stuffed to the brim with stuff than have just 1 book or laptop in it. Maybe try some bike specific backpacks first? this would bear the weight better imo.