I would like to purchase a quick release prong collar for my dog. My question is, what size link should I order for my dog?

Dear Mr. Frawley,

I would like to purchase a quick release prong collar for my dog. My question is, what size link should I order for my dog. Her neck measures 17 1/2 inches. The medium fits up to 18" and the large fits 17 to 20". She is possibly a Rhodesian Ridgeback mix. Her coat is a bit like a Shepherd but shorter. With her ears down she looks like a Rhodesian and with them up, she looks like a Malinois. She weighs 68 pounds and looks a bit dainty, but she is anything but! She can take off and pull like a drag racecar.

Thanks for your attention.

Sincerely,
Randee