The size number is obtained by measuring the chest circumference and then dividing it in half, thus if the chest circumference is 80 cm, my size will be the 40.
Measure your bust, waist and hips to find your size.
If you need help have a look of the picture below.
FIG.1 BUST CIRCUMFERENCE
Don't squish yourself measure all the way around your bust and back right at your nipple line, make sure the tape measure is not too low
FIG.16 WAIST CIRCUMFERENCE
Measure your waist at the smaller circumference of your natural waist, usually just above the belly button.