We have 4 sizes available so please refer to the measuring chart so you get the size most suited to your baby. Harness sizes are not based on weight or age. The size is based on the length of the baby's torso. To determine which size is best for your baby measure from baby's bottom up to baby's chest (underarm height) since the harness goes accross the baby's chest. You can purchase a harness that is longer than this length and roll it downward. Do not use a harness that is smaller then this length. The purpose to crossing the chest is to keep the baby securely in place so that he/she does not sit-up or roll over.