A bike bag is a functional accessory in which size really does matter. Select a carrier that holds what you need without being too big or too small. You don't want your folding allen key set bouncing around or worse, getting lost, in your grocery pannier. It should be durable and water-resistant, if not waterproof, and re-inforced where necessary, whether made of fabric or leather. Fastenings like zippers, snaps, and buckles should be heavy duty and work smoothly.