I ordered a medium size bag.  I commute on a motorcycle and I can easily fit my gym clothes and other daily use stuff in here.  In fact, I was able to carry size 10 sneakers and an actual shoe box, ladies size 8, high heels, in this box to return them to the store for my wife.  In addition, I had all of my bike tools, my toll road pass, phone, wallet, watch, extra stethescope, water bottle, and a Kel Tec P32 in the bag.  I had to close it with the velcro, but it stayed closed at 65mph.
The shoulder strap adjusts easily even with leather gloves on and its long enough to fit over my helmeted head to put on.  The cross body stabilizer works great- the bag does not move at all while riding, walking, etc.  No restriction of arm movements.
Finally, there are reflective strips on the flap.  I highly recommend this bag.  Its amazing.