Most people who sew know that a manufactured dress size and a dress pattern size have nothing in common.  This guide is purely for fun and to show the changes in standard sizing for dress patterns through the years.  It has nothing to do with the little tag on a dress in the department store. Sizes weren't routinely listed on all patterns until around 1940.  Prior to WWII the sizing was strictly related to the bust measurement. Here is a brief look at the changes in a  Size 12  from 1930-2000

1930- Bust 30  Waist 25   Hip 33	1940- Bust 30  Waist 25   Hip 33
1955- Bust 30 Waist 24 Hip 33	1960-Bust 32 Waist 25 Hip 34
1970-Bust 34  Waist 25.5  Hip 36	1975-2000 Bust 34 Waist 26.5  Hip 36

So, as you can see there is quite a difference in the fit, silhouette, and sizing of vintage patterns throughout the past 60 years.

Most Pattern sellers will list the Bust Size in the title of the listing so you can browse at your leisure instead of plowing through multiple listing only to find the pattern won't fit.  An experienced and talented person who sews can always moderate and alter a pattern up or down, but it's nice to know what you are getting from the beginning.