To measure your dog for a Martingale Collar, use a soft tape measure or a piece of string and take the measurements at point A, B, and C in the drawing below. Choose the size that fits within this range to ensure that your collar fits perfectly.
For Sighthounds, the measurement at points A and B may be the same or very similar, this is because their heads are often the same size or smaller than their necks. For non-sighthounds, these measurements will be different and you will need to choose a collar size that is large enough to slip over the head (point A) but not too large to fit correctly at point B.
In other words, choose the smallest size that will still fit your dog. If your dog has a 15 inch neck, choose the Medium, which is adjustable from 13-18 inches rather than the Large, which is adjustable from 15-20 inches.