Measure the exact size of your wrist in the place where you would normally wear your bracelet.
Use a flexible measuring tape (like the ones from IKEA or a sewing store)
or
Wrap a piece of string around your wrist and mark it where it overlaps. Then place it on a flat surface beside a ruler.
then
choose how tightly you want your bracelet to fit, for example:
if your wrist size is 6″
A 6″ bracelet would be a snug fit
A 6.25″ bracelet would be a comfortable fit
A 6.5″ bracelet would be a loose fit