During a dog's life, you may have to buy a few collars, as a dog goes through a few sizes until he is full grown. Select a collar that your dog can wear around his neck, snug enough to stay on, but loose enough to allow you to place two fingers beneath the collar.
With an adjustable collar, you should have little difficulty finding the right fit. Select the type of material most suited to your needs. This can be either a leather collar or a nylon one.
There are some dogs that actually would do better with a harness than a collar. A dog harness relieves stress from around the neck and distributes it on the strong body. Most still have a collar for tags, but the leash is connected to the D-ring on the harness.Breeds that are susceptible to a collapsed trachea should wear a harness instead. It provides better control, and it's healthier for the dog. When in doubt, choose a dog harness. It's safer and causes less stress from pressure on your dogs neck.