A fantastic article published in The Economist in December, 2010 revealed a team of archeologist's findings on human health and height in 15th century England. These archeologists examined a mass grave from a War of the Roses-era battle and determined that the average height of participants was only 4cm shorter than the average height of and Englishman today! That is not much of a difference.
Other researchers suggest that average height did decline during the 17th and 18th centuries, bottoming out at 65.5" (about 5'5.75") at some point in the 18th century. This information comes from studies done on skeletons excavated in northern Europe, not the colonies. Records indicate that the average American soldier during the Revolutionary War was about 2" more than the average height of the British troops of the time.