Measure chest girth at the widest point behind the front legs.
A properly fitted harness distributes pulling force across the chest rather than the neck, reducing risk of tracheal damage. Measure chest girth: wrap a tape measure around the widest part of the ribcage, just behind the front legs. Add 2" for comfort. You should be able to fit two fingers between the harness and your dog. Common sizes: XS (12-16" chest, 5-10 lbs), S (16-20", 10-25 lbs), M (20-26", 25-50 lbs), L (26-32", 50-80 lbs), XL (32-40", 80+ lbs). Front-clip harnesses reduce pulling; back-clip are easier to put on. Check fit regularly — puppies outgrow harnesses quickly.