This is from the Hurtta site....
MEASURING THE DOG
1. The back length should be measured from the withers (not the collar) to the base of the tail, while the dog is standing in normal position.
2. The neckline measurement should be taken at the base of the neck, this being the widest point of the neck. The neckline of an overall can be larger than the circumference of the dog’s neck, as it can be adjusted to fit a smaller neck.
3. The chest measurement should be taken at the widest point of the chest, from behind the front legs. To ensure sufficient freedom of movement, the chest of an overall should be 5–20 cm/2-8” wider than the dog’s chest.
4. The front leg length should be measured from the bottom of the rib cage along the inner side of the leg to the wrist bone. The legs of an overall are of an appropriate length when they reach down to the dog’s forearms.
Note: If the purpose of the overall is to protect the fur of a long-haired dog, the legs of the overall can be longer than listed above. In this case, however, the overall should only be worn when the dog is on a leash or under its owner’s supervision.