Greyhounds, Saluki, Afghans, and other medium sized dogs will need a size Medium martingale dog collar, which is adjustable from 13-18 inches. Some larger greyhounds (80+ lbs) sometimes need a size Large martingale collar, which is adjustable from 15-20 inches. You will be able to determine this when you measure your dogs neck.
Large Greyhounds,smaller Labs ,Golden Retrievers,and other large breed dogs most often need a size Large collar, which is adjustable from 15-20 inches.
Great Danes, larger Labs and other very large breed dogs will definitely need measurements to determine the correct size. Some Danes can wear a size Large, some require an XLarge collar. it really depends on the size of your dog.
Small 10-14 inches
Medium 13-18 inches
Large 16-20 inches
XL 19-26 inches
XXL 26-34 inches