Claim: Marilyn Monroe wore a size 16 dress.
Status: Not exactly.
Origins: The fascination with this "fact" about Marilyn Monroe's dress size is not its literal truthfulness per se, but the implication it carries: that our standards of feminine pulchritude have become so extreme that the woman who has been idolized as the world's premier sex symbol for half a century would be considered "chunky" or even "fat" by modern standards. (Conversely, some of today's celebrities seem to be fond of invoking the "fact" that Marilyn wore a size 16 dress as a means of asserting that they themselves are, if not thin, in better shape than the renowned Marilyn Monroe was.) Marilyn may (at times) have been a little heavier than today's ultra-svelte models, but the notion that she was "fat" (even by today's standards) is based on misinformation or misunderstanding.
The claim about Marilyn's dress size is difficult to prove or disprove, both because of a lack of information and because of the number of ambiguities it engenders:
Like most women, Marilyn Monroe experienced weight fluctuations as she aged, became pregnant, and went through cycles of weight gain and dieting, and so she would not have worn any one dress size, but a range of different dress sizes.
The standards for women's dress sizes have not remained constant over the years; they have changed as the size and shape of the average woman has changed. (Clothing manufacturers assume most women don't want to wear clothing of a size identified as "Large," for example, so they adjust their sizing so that the average-sized woman takes a "Medium." If the size of the average woman has increased a bit over the years, then the very same size that was a "Large" fifty years ago might be a "Medium" today. This is sort of what has happened to women's dress sizes since the 1940s: a woman who weighs more now than she did twenty years ago might actually be wearing a smaller dress size today.)
A person's overall physical contours can't necessarily be determined from a piece of his clothing. A very tall and skinny man might have to buy pants with a waist size larger than he needed in order to get the correct length of inseam, for example, and therefore inferences about his weight based solely on the waist size of his pants would probably be inaccurate. Likewise, a woman whose bust, waist, or hip measurements were unusually large or small (such as a woman with an acclaimed "hourglass" figure) might have to buy a size of dress that was not reflective of her "overall" size. (Even examining the clothing Marilyn wore in her heyday wouldn't necessarily answer the question about her dress size, since her outfits were custom-made by studio dressmakers and therefore didn't bear size tags or hew to standard industry dimensions.)
Reliable documentary evidence is tough to come by for this sort of thing. Doctors might record a patient's height and weight, but not her dress size or measurements; other citings of Ms. Monroe's size are difficult to take at face value, as their sources (e.g., studio publicity shops) are known more for exaggeration and puffery than accuracy. Additionally, weight and other size measurements are mere snapshots: a mature person's height is unlikely to change significantly, but the other numbers can vary quite a bit over time.
So, what can we say with any certainty? We can at least establish a range of measurements for Marilyn Monroe based on the available sources:
Height: 5 feet, 5½ inches
Weight: 118-140 pounds
Bust: 35-37 inches
Waist: 22-23 inches
Hips: 35-36 inches
Bra size: 36D
A woman of Marilyn's height, at the extreme of Marilyn's weight range (140 lbs), would probably wear a size 12 dress today (which is the same dress size listed for Marilyn in the book The Unabridged Marilyn). Perhaps at one time she did wear dresses that might have been considered size 16 (or even 18) back in the 1950s, but she almost certainly did not wear dresses equivalent to today's size 16.
(source:
http://www.snopes.com/movies/actors/mmdress.htm, a site about urban legends)