This plastic boxer dog muzzle has a forehead strap to help prevent the dog from getting the muzzle off. This boxer muzzle will fit short and wide snout dogs. Size 4 The size 4 will fit Boxers and some Pitbulls. The size 4 muzzle has a 14 inch circumference. The length on the size 4 is 2-1/2" on the top and 4" on the bottom. Size 5 The size 5 will fit large Boxers and some Mastiffs. The size 5 muzzle has a 15 inch circumference. The length on the size 5 is 3" on the top and 4-1/2" on the bottom. Measure the length and circumference of the dogs snout. Add 1 to 2 inches to the dogs snout circumference. This is a plastic muzzle and will not hold up very long on a untrained aggressive dog.