One of the most persistant myths about centuries ago is that people were noticably shorter.
The saying usually goes: We know from their armor (or their houses, or their furniture) that people were shorter back then. This gives the impression that everyone was noticeably shorter in the 17th century and earlier. This belief is based partly on old evidence and partly in misinterpreted facts. Examination of the recently recovered 16th-century warship showed an average height of 5'8" inches among the drowned sailors. This is identical to today's average male height in the U.S. (American women are an average 5'4".) This was a big surprise to the Mary Rose researchers, who had assumed from the low ceiling below-decks that the men were shorter. Many of the skeletons were as tall as 6'.
Just because the average height is 5'8" (or whatever - no such average is available for the 17th century) does not mean that everyone was that height. To look at a single 17th-century family, Charles II is known to have been over 6', as was his brother James II. Their great-grandmother Mary, Queen of Scots was about 6' tall. However, Charles and James's father, Charles I, was about 5'4" or less (he was Mary's grandson) and his wife Henrietta Maria is thought to have been less than 5' tall. Charles and James's cousin Prince Rupert (their father's sister's son) was 6'3" or 6'4", while their cousin Louis XIV (their mother's brother's son) started a fashion for high heels and tall wigs to hide the fact that he was only 5'2" or 5'3". So there you have, in a single family, height variations among close relatives which can easily be matched today.
How did such a belief get started? Doing thorough research into this questiron is beyond the scope of our local library facilities, so I can only offer speculations.
Misinterpretation of physical evidence is a major factor. We are used to 8' ceilings and, if an older house has a 7' ceiling we are told, "They were all shorter back then." (Interestingly, the reverse explanation is never used in a house with a 9' ceiling.) Also, in centuries past, houses had much lower doorways, not because their inhabitants were shorter, but to reduce the circulation of cold drafts. (References from several periods and social classes support this.) The same is true with beds. In prior centuries people habitually slept semi-reclining, propped up on bolsters, and their beds were shorter. Numerous illustrations from the 14th-17th centuries bear this out, but guides in historic houses still give the same explanation: "They were shorter then."
Clothing and armor surviving from earlier periods do not give reliable guides to people's average body size either. Most surviving 19th-century ladies' shoes are those left from window displays, and were the smallest sizes. Likewise, 19th-century kid gloves were meant to be tucked into a belt or carried, and are much too small for actual wear. Many people who saw the "Tower of London" exhibit and saw Charles II's suit of armor probably thought he was only 5' tall, although it was made for him as an adolescent. At Pilgrim Hall in Plymouth, Massachusetts, are displayed a chair and a sword which, according to tradition, belonged to Miles Standish. The tradition rests partly on the strength of the fact that Miles Standish WAS short, and that the chair and the sword have been cut down at some time. Although Standish is known to have been called "Captain Shrimp" by a detractor, unless he was less than 4' tall it's EXTREMELY unlikely that he'd have needed to have chairs shortened for him. (A more reasonable explanation is that both items were damaged and made useable by being refinished, shorter; the sword may simply have been made over into a more fashionable length later.) Surely contemporaries would have remarked on it if Miles Standish had been a midget!
Even when presented with clothing or armor from a man around 6' tall, most people tend to dismiss that individual as being unusually tall, while accepting the articles of a shorter man as normal.
Another contributing factor to this commonly held belief may have begun in the 18th century, when there was a widespread movement to drinking gin instead of beer. Since gin has no food value, although beer does, this led to widespread malnutrition among the English. Poor nutrition can indeed result in a shorter population. Later in the 19th century, when nutrition was improved, people would have looked back at their parents' and grandparents' generations as shorter and might have extrapolated this back into previous centuries.
One further possibility is that the traditional view of history causes a distortion in itself. History usually follows the "highest" civilizations, ignoring all others. Thus history starts in Egypt and proceeds through the Middle East to Greece and Rome. Even today, those areas are known to produce shorter inhabitants (shorter than modern Americans, at least). It was always the barbarians who were taller. The centers of modern civilization, however, are made up of descendants of the barbarians.