Properly fitting a harness or collar is crucial for your dog's comfort and safety. A collar should fit snugly, allowing two fingers to slip comfortably between the collar and the dog's neck. For harnesses, ensure an even snugger fit across the chest and back, while still allowing comfortable movement. It's important to adjust the harness to prevent any chafing or restriction of movement, particularly around the shoulders. Regular checks and adjustments are needed as your dog grows or changes in weight to maintain the perfect fit.