It depends on what you need the bag for. If it's just to run around town or go out a night, something just a couple of inches deep is enough to hold the essentials. If you need to hold a change of clothes, snacks, and a tablet, you'll want to look for dimensions closer to 7 x 9 inches. Go too much bigger, and you're getting into a territory where it might not be comfortable to have the weight hanging from one shoulder.