There's a frequently-encountered assumption that the average height of humans has grown steadily taller as the centuries have progressed. However, studies have shown that the average height of a population is related to general health and economic well-being, which is affected by such factors as climate changes, the growth of cities, war and population cycles. Thus, average height fluctuated throughout history.
And it turns out medieval men weren't a whole lot shorter, on average, than men of the new millennium.
A recent study conducted at Ohio State University, based on skeletal data from 30 previous studies, reveals that men living during the 9th to 11th centuries had an average height of about 5 feet 8 inches. Average height then steadily declined until it reached a low point of 5 feet 5.5 inches in the 17th and 18th centuries, rising again through the 19th century and only reaching prior heights in the first half of the 20th century. An article on the study by Richard Steckel appears in the Social Science History journal.
According to the feature by Jennifer Warner at WebMD, Dr. Steckel noted that "Average height is a good way to measure the availability and consumption of basic necessities such as food, clothing, shelter, medical care, and exposure to disease." Frequent wars and a rise in the spread of deadly diseases impacted population health and height, as well.
Today, average height in American men is about 5 feet 9 inches and in American women 5 feet 4 inches. According to a study completed in 2002 (see the article by John Carvel at the Guardian), Britons have similar average heights, while most Europeans are an inch or two shorter. The exception is the Dutch, who are about an inch taller than Brits and Americans.