The centre of these fins flexes inwards to fit people with narrow calves and/or thin legs. So even if your legs are very thin and/or your calves are very narrow, the fins will still fit you.
They match your leg shape by bending around your shin. You can adjust the fin height on your leg for the most comfortable fit.
The straps go around the narrowest part of your leg, just above your ankle. So please measure the circumference around this narrowest part. If this circumference is 20-40 cm (8-16 inches) then the standard straps supplied with the fins will fit you.
If this circumference is less than about 20 cm (8 inches), then the fin centre may not bend inwards quite enough. Then you may like to wear your fins over a wetsuit. The wetsuit effectively thickens your lower leg.
See all FAQ about the adjustable fit of leg fins.