Sizing is not standardised.

Even that would give no definite result, as the fit plays a roll as wel. A fit for small sized people, ie Italian, French, or a fit for larger people, north europeans, Germans.
Then there is the manufacturer. Hand made in sweat shops, to the specs on the work floor there.
It's a simple
Why sellers take the trouble to advertise but leave the most important details out i will never understand.
"For sale: Car."
"For sale: skirt." or "for sale: skirt size L. " No colour, no material, no type, no length, no internal measurements.
You would think that those people have never bought anything themselves.
If you have a stable weight,

and you have measured the ACTUAL waist and hip size, that you have when wearing any garment that "fits" you comfortably, then you can use those measurements to "size-up" any garment that you cannot actually fit before buying.
Then always ask for the actual inside waist and possibly hip size. And more measurements like your breast width etc. if you are buying a dress, or shirt. You should ask for the only half-measure, which you get when skirts are laid flat. When done seriously, and securely it should be OK. That being half of the measured size you took from yourself. That should never give any problems, unless the seller is selling products which he/she does not have in hand, or are packaged.
That always gives a very good indication for correct sizing. But as with myself, there is some ?leverage? in sizing, as I am getting a little bit flabby in the waist, which sometimes results in the possibility of buying a skirt in two sizes. One being tight and the other loosely fitting. Know yourself, and buy on experience. Measuring is knowing.
A man is the same man in a pair of pants or a skirt. It is only the way people look at him that makes the difference.