Chain bracelets
Chain bracelets are sized by total length, usually 17 to 19 cm for retail, with an allowance over the wrist measurement. Measure the wrist snugly and add the allowance agreed with your market.
Include the clasp in the length and state it on the spec; a pattern that looks right in a photo can be 5 mm short in the hand.
Bangles
Bangles are sized by inner diameter because they have to pass over the hand. A slip-on bangle needs a diameter larger than the widest part of the hand, not the wrist.
Hinged bangles behave like a chain bracelet for fit and are easier to size across a range.
Tennis bracelets
Tennis bracelets are set link by link and sized by length; links can usually be removed to shorten but not added. Order slightly longer where in doubt and plan a link-removal service.
Confirm clasp type and safety catch on the sample, since a bracelet is the piece most likely to be lost.