While it's best to opt for well-fitting cuffs and bangles, choose slightly longer links and chains to give them more movement.
Take a measuring tape, a length of string or a strip of paper.
Wrap the ribbon/string/band around the wrist you plan to wear the bracelet on. Make a mark where the ends meet. Are you thinking of accumulating bracelets? Take your measurements at the location on your arm where you will wear each bracelet.
Lay the string or paper on a flat surface and use a ruler to measure the length to the marked mark.