Shopping for vintage definitely has its perks - one of kind items and it’s good for the environment. The one downside is sizing. Sizes have changed so much throughout the decades - not to mention brands have different size specs.
So how do you know what size to choose? Your best bet is to buy a measuring tape and take your own measurements. Another thing I would advise is to take something you already own that fits you perfect, and measure that.
How measurements are taken
I measure all my garments on a flat surface, lay the item flat, and measure from seam to seam on all of the below points.