The overall circumference measurement of your dog's head 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.
|Size||Max Width||Max Height|
"I can highly recommend the muffs having taken my search dog on a Chinook. Previously he was a very reluctant passenger on helicopters. This time he pulled me onto the aircraft and appeared very relaxed during the flight."
"Just to let you know my order arrived today. My dog is wearing then now and hardly bothered by the fireworks.
First time in 11 years he has not been climbing the walls at this time of year."