Hi, It's a little complication, but watch the video in the first link listed and it will become clear. (I bought a 20" one for my Springer Spaniel in December and we've both been really happy with it.)
http://www.cleanrun.com/index.cfm?fuseaction=product.display&Product_ID=2997 - below are the directions from here; if you go to the site, you have to click on the left panel that says "Sizing."
How to Measure Your Dog: To fit your dog for a Hurtta Waterproof Fleece Jacket, you first need to determine his back length (measurement #1 in the diagram). If you are deciding between two sizes, use the neckline (#2 in the diagram) and chest/girth measurements (#3 in the diagram) to help you choose.
Be sure that your dog is standing squarely, with head up, and is looking straight ahead. Following the contour of the dog's back closely, measure from the top of the dog's withers (the point of his shoulders) to the base of his tail (the point where the tail joins the body). Do not measure from the dog's collar.
The neckline measurement is taken from the widest part of the dog’s neck; that is, from the base of the neck. The neckline of the jacket may be several centimeters larger than the dog's actual neck measurement since the neckline of the jacket can be adjusted smaller.
The chest/girth measurement is taken from the widest part of the dog’s chest, just behind the forelegs. Breeds with long and thick fur need more room for the chest than dogs with short fur.
If you are still uncertain of sizing, please call or email us for assistance.
Size 12"/30cm: Miniature Schnauzer, Chinese Crested, Kleinspitz, Pug
Size 13"/33cm: Bichon Frisé, Jack Russell Terrier, Coton de Tuléar
Size 14"/36cm: Border Terrier, Poodle, Pinscher
Size 16"/40cm: Beagle, Cocker Spaniel, Shetland Sheepdog
Size 17"/42cm: Miniature Dachshund, Dachshund, Drever
Size 18"/45cm: Australian Kelpie
Size 20"/50cm: Wheaten Terrier, Nova Scotia Duck Tolling Retriever
Size 22"/55cm: Australian Shepherd, Labrador Retriever, Dalmatian
Size 24"/60cm: Bouvier, Rough Collie, German Pointer, Golden Retriever
Size 26"/65cm: Boxer, Rottweiler, Doberman, Giant Schnauzer