Use a paper measuring tape to measure around your wrist. If you do not have one, you can use a piece of string or a marked paper to wrap around your wrist and then measure the length with a regular ruler.
If You Don't Have a Measuring Tape
Use a strip of paper or a piece of string instead. Wrap it around your wrist and mark where it overlaps with a pen. Then, measure the marked length with a standard ruler to determine your wrist size.