I got a Sugarloaf bag back in June. I love this bag! It fits perfectly within a Wald basket (medium) with straps that hold it into the basket. They fit perfectly. I bought it for two purposes: commuting and also bike touring.
I went on a S24O on the GAP back in June and used this bag. I stored my clothes in this bag. A pair of shorts, a t-shirt, my second days riding clothes and my Dopp kit. It easily held all of this. I also had a medical kit in the front pocket. It worked great.
I ride nine miles between Catonsville and Locust Point in Baltimore. It easily holds a days clothing and my lunch and still have room to spare. You could store your shoes also if needed and still have room to spare.
For either task, this is an amazing bag. It holds a decent amount of stuff. As a commuting bag it can hold all of your clothes and your lunch with room to spare. As a touring bag, it can hold 2-3 days worth of clothes with room to spare unless you are wearing a suit or something.