The short answer is: Measure Yourself!
Stores with an online presence will usually include a Size Chart on the web page for a given item. Knowing your measurements will allow you to compare against the chart. Often there are instructions on how to take your measurements. All you need is a cloth tape measure.
Some web sites, like TJ Maxx for example, will cheat and post a generic Size Chart, so that does add a bit of risk.
Also, be aware that Size Charts do not take into consideration how the designer intended a garment to fit. Clothes can be traditional fit, bodycon, oversize, etc. so that adds an additional layer of complexity.
Actually, there is reasoning to it, but there are no standards and consistency is lacking. Over time there has been some of what you might call “slide”. By that I mean manipulating the size designations to give women the sense that they are smaller than they actually are. Would you rather be told that you fit into a size 14 or a size 12? That is also the thought process behind the sizing plan that Chico’s and other companies have done. Their size chart is: https://www.chicos.com/store/page/size-chart/45. In dresses, I usually wear a size 18 or 20. Would you rather be told that your size is 3.5 or 4 instead of 18 or 20? That is just mental masturbation.
Anyway, don’t expect consistentency between manufacturers and even within the same manufacturer. How do I know this? I have 5 dresses by Sharagano; same style, same fit, same fabric composition, different colors. Depending upon which one you look at, the sizing is 16, 16W and 18. How’s that for consistency?!?!
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