How do I know what size to get? Measure the dog's neck where its collar rests naturally. You should be able to put two fingers between the measuring tape and the dog's neck. Having the correct size is important for the Puppy Bumper to be comfortable and effective for your dog. How do Puppy Bumpers stay on? Puppy Bumpers have ribbon loops on the inside of the neck hole. A collar (not included) slips through the ribbon loops to hold the Puppy Bumper on securely. We suggest having a separate collar that you can keep with the Puppy Bumper.