To ensure a proper fit, I recommend you use the please use the following guidelines.
1.   Using a piece of string or a 1/4" wide strip of paper Wrap it around your wrist just below the wrist bone. The fit should be snug but not tight.
2.  Make a mark on the string or paper where it overlaps to form a complete circle around your wrist.
3.  With a ruler, measure the length of string or paper to determine your wrist size.
Consider how snug or loose fitting you want your cuff.  Add the extra length to your total.
Cuffs are intended to be worn snug around your wrist with a 1″ opening.