Chain collars, also known as choke collars, are training tools, not meant to be a substitute for your dog's regular collar. Since this type of collar can inflict damage to your dog's neck and throat if used improperly, it's vital to choose the correct size so you can use it in a manner that won't hurt your dog.
Place a fabric measuring tape where the chain collar will sit around your dogs neck. This is usually about the midway point of his neck. Pull it snug so there is no gap at the bottom, and note the measurement. Add 3 inches to the measurement since training collars should not be snug. If you don't have a fabric measuring tape handy, take the measurement using a piece of string and then measure the string length using a ruler. Choose the collar length that is closest to the total after adding 3 inches. Select the next standard size up, rather than down, if your measurement falls between standard sizes. For example, if your total measurement is 18.5 inches, and the collar sizes you find are 18-inch and 20-inch, select the 20-inch collar.