Size?  What size?   Hmmmm?
Typical Maximum Muff Dog Head Inner Shell Size Weight Circumference Width Height X-Small 5-10 lbs 7-9" 1.5" 1.625" Small 9-25 lbs 9-13" 1.75" 1.75" Medium 20-50 lbs 13-18" 2.5" 2.5" Large 50-95 lbs 18-23" 3.25" 3.25" X-Large 95+ lbs 23+ 4" 4"

The overall circumference measurement of your dog's noggin at the largest point is the best determination of size, because if you choose a size too large, the muffs won't seal well against his head, and you lose all the sound-reducing properties you were shooting for.  Your dog's weight is also a good indicator of which size to choose.