Myth: Marilyn Monroe was a size 12-16. She was very big”, you’ll often hear people saying Marilyn Monroe was around the same size as the average American woman today (12-16). So what size was Marilyn Monroe actually? Hugh Hefner purchased the burial vault next to Marilyn Monroe as his future place of final rest. Marilyn Monroe started going by that name in 1946 around the same time she dyed her hair blonde and divorced her husband.