SMALL is great for smaller ladies' watches or children's watches. If you plan to attach your ID to your shoe, zipper pull, key ring, etc., you may want to consider a medium to fit more information on your ID.
MEDIUM is the perfect size for most watches, dress watches, or sports watches. Medium is also an ideal size for when you attach your ID to your shoe, zipper pull, key ring, etc. For most uses, medium is a great option.