In the early days (11th, 12th and possibly into the 13th century), there probably wasn't much difference between the battlefield lance and the battlefield spear. A first, the spear used by cavalry was almost certainly the same as that used by the infantry, as manuscript miniatures and the Bayeux Tapestry show no real difference in length or spearhead. Especially during the 11th and early 12th century, the lances might also double as javelins, though dedicated javelins of a similar length, but thinner shaft, may have been used as well.

As armour improved and the couched lance became the only form of spear use on horseback, the shaft of the lance probably thickened and became more durable. Up until the mid-14th century, the lance likely did not exceed ten feet in length, even as it became thicker. As the use of the lance rest to hold and stabilise the lance and a backstop to impart even more energy into a lance strike came into widespread use, the lance became longer and tapered heavily from the grip to the point. The length was around the fourteen feet mark at this point, and didn't change much during the remainder of the medieval period so far as I can tell. Unfortunately, the evolution of mounted combat isn't very well studied beyond the initial transition from the multipurpose lance to the couched lance, and even that contains a large amount of speculation and debate.