For eyeglasses that you want to fit snugly against your head, measure around the part of your head directly behind your glasses stems. The sleeve should extend about one-half inch onto each stem. Look for adjustable retainers that give you a couple of inches of wiggle room to achieve the perfect fit. If you want your shades to dangle from the retainer, look for one between 15 and 20 inches long. The shorter ones will keep your specs around your neck, while the longer ones will hit most people in the chest. If you’re not sure, go longer. Many retainer sleeves can be made shorter by slipping them farther up the eyeglass stems.