Inseam isn't your pant size. It's snug in the crotch to the floor. I don't care if you wear shoes but don't have stiletto's on. Most will want no shoes.
I use a carpenters steel tape measure. Holding the body of the tape measure snug in my crotch and aimed at the floor, I extend the tape to the floor. Then I add the width of the tape measure housing which is usually 3.25 inches.
There is no standardized size for what mfr's call S, M or L or even centimeter sizes or inch sizing. So unless you can try it on you'll probably should go by the manufacturers guidance until you know for certain. And that will be pretty hard to do over the internet.
Perhaps call their phone number and talk to the rep about your concerns. I'm sure it's probably in their top 10 of questions ask by prospective customers.
And they can tell you how they expect your body to be measured.