How long were the heavy lances used by medieval men-at-arms on horseback in the 14th and 15th century? Contemporary artwork provides some evidence, when the composition suggests that there was no need to distort the size of the lance to fit it into the picture. Skeletal evidence gives an average height for medieval English males at 5’ 7.5”. It seems likely that the upper classes, with better than average nutrition in childhood, were about an inch taller.
Scaling to an average height for a man at arms of about 5’ 8”, the lances in these paintings would have the following approximate lengths:
Lancelot du Lac MS, ca 1380 9.2-9.4'
Altichiero, Saint George Chapel, Legend of Saint George 1378-84 9.5’
Roger Van der Weyden, Saint George and the Dragon c 1432 11.5’
Paolo Uccello, the Rout of San Romano c. 1450 11.5’
One further piece of evidence: Giovanni dall’Agochie, writing in 1572, recommends a lance length of ten feet.
It would appear that lances became significantly heavier, both thicker and longer, after lance rests came into common use, beginning around the end of the 14th century.
Some additional information on 16th c. lance dimensions is available here.