The length, width, loop size and clip size all matter. While a 6 foot, 1/2 inch, leather leash may be perfect for a 100 lb German shepherd, it’s not a great choice for a Bichon Frise. Finding a thin, 3/8″ wide, nylon leash with a small clip is best for small dogs. The length is also important if you’re working on training your dog not to pull on leash. No sense in constantly winding the leash around hand if you’re trying to teach your dog to walk nicely next to you. In that case a 4 foot leash is better for training.