Measure with a flexible tape measure to get your wrist measurement. You can do this with a strip of paper or string if you need to, mark where the ends meet, and then measure between the marks on a normal ruler.
Take your wrist measurement and add about 1 inch, depending on the bracelet. A thin weighted bracelet, such as the Petite Byzantine, would only need 3/4 to 1 inch, while the chunky Bold Byzantine would need a 1 1/4 inch. If in doubt, leave your exact wrist measurement in the comments when you checkout and I will size it appropriately. Let me know that it is your wrist measurement, and not your bracelet length.
Here is a printable ruler PDF you can print out -- make sure you print it at 100% with no scaling, or the ruler will not be accurate.
If even after careful measuring, you find you need your piece resized, please see my exchange/resizing policy.