Sizing research

I am working on a block pattern for a skirt in woven fabric and I’m struggling a bit with the sizing. Even though I provide measurements I also want to have a sizing system that comes close to my customers expectations of a medium or whatnot. My customer is a grown up woman, hence a small is bit larger than shops catering towards teenagers. And my XS would be for the slim folks (though I don’t have that size yet as I never get any requests for it, only XL).

Today I checked out skirts in retail chains such as Mango and Zara. However I think their Mediterranean sizing is a bit smaller than the US market equivalent (where most of my customers are). I’m wearing a Zara medium in the photo, and it fits pretty well, but maybe some would find this skirt a tad snug for being a medium. I think that my lower body falls somewhere between a “generic” small and medium. What would you say, does this skirt looks like a medium to your eyes or is it more a small? I know it might be hard to tell from a photo, but for reference, I wear a size 6 in US brands like Anthropologie, Ann Taylor loft and Banana Republic.