More about fitThere is no standard that vintage sellers collectively use to describe sizes, but let's be fair, there is no precise standard for modern clothing either!
I have tried to at least codify my own size estimates, as you can see by looking at a previous blog post Women's vintage clothing: determining size. Other sellers have done the same. However, even in my listings I would recommend that if you see size 6 say, or XL, do not assume it is the same as your idea of size 6 or XL. Go straight for the measurements and compare those to your own.
Many sellers, including me, suggest that you compare the measurements of an item you're interested in with something similar of your own that fits you well. This is great if you indeed have something similar in key ways, for instance non-stretchy if the item you want is woven not knit, and fitted in the same ways. I will later make suggestions about how to choose items even if you have nothing similar.
When should you ask for further help from a seller? By all means ask if there are no/not enough measurements or the condition is either not listed or described. If a seller offers almost nothing they may either be beginners or just not very skillful sellers.
One characteristic of a good seller is that they offer most or all of the information you need to make an informed decision in the listing. Most good sellers will be happy to guide you if you need further help with the size or some other aspect. Just remember, don't ask if that 1950s dress is a size 6...ask about the item's measurements.
Now that you've taken a look at knits, you are ready to delve into woven items, items without much or any stretch. Here fit is so important.
A great beginner's vintage item is a flaring coat or jacket. These are chic, stylish items dating most often from the 40s-60s, and working for a range of sizes, even sometimes maternity. The most fitted dimension may often be the shoulder width. Think of them as just one step more fitted than a cape, which you can see also makes a great vintage choice.
50s pink satin swing coat at violetvillevintage, 60s tartan cape coat at gogovintage, 60s navy cape coat at alexsandras, 40s-50s red swing jacket at badgirlvintage, 50s black swing coat in my own shop
Next time: Still more about fit!