A typical dog leash should be 6 feet long to give your dog ample length to walk in front of you without pulling you along. If you are in the process of training your dog, such as training your dog to walk at your side and obey "walk", "heel" "sit" and "stay" commands, you should use a shorter leash of about 3 feet in length. In that case, you can still use the same 6 foot leash, just wrap in around your hand to shorten.
They also make 15 - 16 foot leashes and these are also used primarily for training (i.e., letting the dog out along the leash, calling "come" and being able to reel the dog back in).
But for general, all purpose walking, a six foot leash is standard.
If you are trying to train your dog not to pull on the leash, the best and most effective method is to use a special leash called a "Halti Leader". The Halti fits over your dog's face but does not restrict the mouth (it is NOT a muzzle). It works just like a horse's bridle but does not use a "bit" in the mouth. The Halti allows you to steer your dog by the head instead of the neck. They are designed to eliminate pulling and lunging and are VERY effective.
Source(s):
http://www.lucy-the-dog.com/haltileader.html