Use a piece of string to wrap around your neck where you want the necklace to sit. Use a ruler to measure the string length. That length is what you should order. For example, if your measurement was 13.5", then you would order 13-14".
Use a piece of string to measure around your wrist. Add 1/4" to that measurement for a snug fit or add 1/2" for a comfortable fit. If you're purchasing as a gift, you may want to consider adding a 1" extender to the bracelet for an adjustable fit.