Proportions are really wild in getting a right fit, like, I'm 5'9', 185lb, and I am a 16-18 petite on pants, and around a 22, or XL on blouses, and 16 or XL on dresses. I go to thrift stores or department stores in jeans and a blouse, with my hair in a pony tail with light makeup and jewelery, like on Monday mornings around 10am when few people are there, and when they have put out their new stock. Nobody really cares, ha, even here in small town Georgia.
Now here's my trick: I bring a tiny tape measure, and, knowing my waist, chest, waist, torso length, and inseam, I measure directly, because there is a large variation in sizes. This practice, especially at thrift stores, has been a low cost way of getting a feel for my size for when I order online. Amazon has a good size chart that cross-references the different measurement formats, including European, and expresses sizes in inches.