This is the size chart for Hurrta. I think the overalls are more for keeping pups dry rather than warmth. Katie has the Hurrta raincoat and the waterproof fleece jacket, and both keep her very dry. This overall has been discontinued, so there are places that have it heavily discounted. GW Little has it for $37.00, but the largest size is the 14M. It has been replaced with the combat suit.
Hurtta dog coat size chart and how to measure your dog. Dog Overalls for Outdoors - by Hurtta