I, too, have heard of this. The fact that Marilyn (whose waist was 22-23 inches and whose hips were 35-36 inches, according to most sources) wore a size 14 back then proves that they have changed sizing A LOT. This is sizing chart for women's clothing in general:
http://www.wmsclothing.com/sizing.html

By today's sizes (and Marilyn's hip measurements), she would be wearing no larger than a size 4. Soooo a 1950's 14 = a 2000's 4. Even the EXTREMELY slender Audrey Hepburn wore a size 8 back in her day. Based on her measurements of 32-20-35, she would be a size 2 (or smaller) today. Ouch.

Just thought I should burst everyone's bubble, haha.