In the words of Albert Nimzicki, "That's not entirely true."
The Hollywood Movie Museum has fifteen of Marilyn Monroe's dresses, which range, according to the caretaker, from sizes 4 to 8 by today's sizes. The white dress she wore in "The Seven Year Itch" is a size eight.
However... dress sizes are based on bust and hips measurements. While Marilyn said her epitaph should read "Here lies Marilyn Monroe, 38-23-36", there is some argument that she wasn't really that big. But assuming those measurements were true, and the fact that she was just under five and a half feet tall and weighed between 118 and 140 pounds (the upper weight from when she was pregnant and filming "Some Like It Hot"), finding a regular dress that fit was somewhat of a task. Almost all of her dresses were custom made. And custom made dresses don't have sizes.
Today, a five-and-a-half foot woman weighing 140 pounds would most likely wear a size 12 dress. However, most five-and-a-half foot women do not have Marilyn's measurements (i.e. brimming with nice delicious breasts). Still, by all accounts, Marilyn Monroe did not wear what would be considered a size 16 today.
There is another problem, in that dress sizes have changed over the decades. What was a size 12 in the 1940's is now a size 8, because women on average have gotten larger, and because women feel more comfortable buying dresses in smaller sizes. (Think about it: If you fit into two identical dresses, one a size eight and one a size six, which one are you going to pick?). So, in that respect, Marilyn may very well have worn a dress with a "Size 16" tag on it.