A harness should fit comfortably, but make sure you can pass two fingers between the harness and your pet's body (and remember to regularly check under the harness for redness or hair loss that could be caused by an ill-fitting harness). Remember, young pets grow fast and their harness will need adjusting regularly and eventually replacing. For your pet's safety, examine the harness regularly for wear and tear and replace when appropriate, remembering also that pets can chew items unexpectedly, and if any material is ingested to seek immediate veterinary advice. Do not leave pets unattended while using this harness, and please ensure you remove your pet's harness before placing them into a pet crate. Did you know it is a legal requirement to ensure your dog is suitably restrained whilst driving so they cannot distract or pose a risk of injury. Do not leave your dog unattended whilst wearing the car harness. Pets can chew items unexpectedly, remove immediately if torn or damaged. Seek immediate veterinary attention if any materials is ingested.