For the fit of support socks to be perfect, there are three measures to be taken from. The circumference around the ankle where it is the narrowest and the circumference around the calf where it is the widest.
The third size is a shoe size which is not so important for the fit stocking The shoe size is just a fingernail to which support stockings you should use.
The most important measurements are Ankle and Calf.
The measurements should be taken immediately in the morning before the legs / feet swell. If you do not find the right measure, start from your shoe size