A good way to measure your wrist size is to use a soft tape measure or a piece of string. Roll it around the larger part of your hand (at your thumb's last knuckle level). Choose which size fits best for your measurement. If you are unsure of which size to pick, go with the larger one so you feel more comfortable.
A good way to measure your finger size is to use a ring you already wear. Just measure the inside diameter of the ring with a tape. The size chart will give you an approximate of what size to choose according to your measurement.