If you have an REI nearby, you could take your typical stuff and jam it into a real bag to test the fit. The Ortlieb website also lists other dealers where you could do the same. If you're out in northwestern Minnesota, and can't get to a dealer, find out what the cubic inches are for a backpack like what you've been using. The medium Office = 793 cu in, the large Office = 1282 cu in, the downtown = 1098 cu in.