This is easy.  Just measure from the dog's collar to the base of their tail.  Most coats and sweaters are sold in increments of 2 inches....10", 12", 14" etc.  Getting a garment that is a little longer is best for warmth and to account for shrinking.  Since you can shop with your pet at Pet Quarters, trying these items on in the store works best, and you leave knowing the fit is proper.