How to Size your Dog Collar
Before you begin looking for dog collars, it is important to know what size your dog's neck is. The easiest way to measure your dog's neck size is with a flexible measuring tape. A shoestring and measuring tape or ruler works in a pinch, too.
Take the shoestring or measuring tape and wrap it around your dog's neck between the ears and the collar bone, where your dog will wear his collar. Add 2 inches to this measurement, and you have your dog's neck size.
When you put your dog's collar on, you should be able to fit two fingers underneath, between your dog's body and the collar. You want the collar to be tight enough that it won't slip over your dog's head, but loose enough that it won't choke or strangle your dog.