Grab a cent, measure your hip size, then divide it by 2.54.
Round the resulting value to the nearest even number - and this will be your bikini line size. For example, for a 96 cm hip circumference: 96: 2.54 = 37.79 -> the lower size is 38! ...
of course, that in many cases the lower adjustable side is made with binders so you can adjust it to size.