Use a flexible measuring tape or strip of paper below the wrist bone.  If using the strip of paper mark where the end of the bracelet needs to be and then measure your strip.  For a snug fit add 1/4" to 1/2"; for a comfort fit add 3/4" to 1"; for a loose fit add 1 1/4".  Keep in mind that the thicker the bracelet the tighter it will fit.  I will be glad to help with any measurement questions you have, here's how to contact me!