The actual size of someone who wears a size 16 in clothing items depends on the brand of clothing, but the average measurements constitute a 41.5 inch bust, a 34.5 inch waist, and 44 inch hips. A size 16 translates to an "Extra Large" in most American clothing brands.
A size 16 is about the size of today's average American woman, who is between a size 12 and 16. However, sizing varies not only by brand, but by time period and by country. In the 1980s, the U.S. Department of Commerce changed the sizing system to make it more generous, appealing to people's desire to wear a smaller size. As a result, a size 8 today would constitute a size 14 or 16 in the 1950s.
Marilyn Monroe was once upheld as an example of a classic size 16 woman, only her measurements did not constitute what is today considered a size 16. She was much smaller, with her size aligning more with an American size 4.
In the U.K., vanity sizing is a bit less common, with an American size 4 averaging a size 8 in England. The average British woman is considered to be a size 16. A woman's size, however, is not indicative of her degree of health or overall physical fitness.