Leashes come in many shapes and sizes, and there's a big difference between picking out a leash for a Chihuahua and one for a Great Dane. 3 to 4 inch leashes are great for young dogs and puppies, or if you're working on training your pet. Longer leashes, 5 or 6 inches long, are generally better for larger or older dogs. That said, it also depends on the environment you're going to be walking in. Shorter leashes are preferred for crowded streets, while longer leashes might be more appropriate in spacious areas with less people.