First and foremost, train your dog to walk properly on a conventional leash. Don't buy a retractable leash until they have mastered conventional loose leash walking. This is important.
Believe it or not, a retractable leash will actually encourage a dog to pull. If an untrained dog pulls on a retractable leash they get rewarded with more space and freedom. If a dog on one of these leads stops pulling, they feel the leash and collar pulling back on them, which can feel like a punishment – so guess what the dog will continue to do?
An extendable retractable leash is for after you have walked your dog to the park and you have given it a release command, your dog can roam and smell the ground etc. while you still have control of your pet. It is for his safety.
They come in 2 sizes
3 m or 118 inches for small to medium-sized dogs
5 m or 196 inches for medium-large sized dogs