Fit is always a bit of a challenge when buying clothing—new or vintage—online. For help taking the mystery out of buying vintage clothing, I've developed some basic information.
Top tip: Check the measurements of the vintage garment you're interested in against something that fits you well and is of as similar a design as possible.
I give size estimates (always the letter size and sometimes U.S. numerical sizes) and my estimates are based on an average of a handful of catalogs and websites...I hope these are good estimates to help pinpoint the fit. I have the experience of putting on a lot of vintage garments, and so I have a first-hand idea of how an item might fit. This is the chart I look at to predict sizes:
One thing's for sure, fashion has dictated different fits through the years. For instance, a 1950s dress generally has a small waist relative to the bust and hip measurements, as compared to many other decades.
I usually estimate the size based on the most fitted dimension. In many cases it is the waist, but often it is the hip, the bust, or even the shoulder width! It sometimes helps to be friends with a person who can do alterations.
A few details about my fit estimates:
I measure the garments flat but don't just go from side to side. I try to take into account the contour, especially at the bust, which can make the measure larger than the side-to-side measurement.
With most regular-fit blouses I estimate the size based on the wearer being about 4" smaller at the bust than the blouse. This goes down a little toward smaller sizes, and up a little toward larger sizes.
With knits, I try to think how the item would look best worn. In some cases I really see stretching the knit as being "the look," in other cases I think the knit should flow more loosely.
With shoes, I lay the measuring tape flat on the insole of the shoe if at all possible. The length is toe to heel, the width is at the widest part of the ball of the foot. Remember this is the widest part on the insole, the leather or other material is wider at the center of the shoe.
With coats, I try to envision how much might be worn underneath and often give a wider range of sizes because so often coats are more free or loose in cut.
With pants and swimwear, I give every possible measure I can think of because I know these are the trickiest to fit accurately.
Always feel free to ask for further measurements, guidance regarding the size, or anything else!
(Modified from my nearly decade-old eBay About Me page.)