There is dispute in medievalist circles over the size of the war horse, with some notable historians claiming a size of 17 to 18 hands (68 to 72 inches, 173 to 183 cm), as large as a modern Shire horse. However, there are practical reasons for this dispute. Analysis of existing horse armour located in the Royal Armouries indicates the equipment was originally worn by horses of 15 to 16 hands (60 to 64 inches, 152 to 163 cm), or about the size and build of a modern field hunter or ordinary riding horse. Research undertaken at the Museum of London, using literary, pictorial and archaeological sources, supports military horses of 14 to 15 hands (56 to 60 inches, 142 to 152 cm), distinguished from a riding horse by its strength and skill, rather than its size. This average does not seem to vary greatly across the medieval period. Horses appear to have been selectively bred for increased size from the 9th and 10th centuries, and by the 11th century the average warhorse was probably 14.2 to 15 hands (58 to 60 inches, 147 to 152 cm), a size verified by studies of Norman horseshoes as well as the depictions of horses on the Bayeux Tapestry. Analysis of horse transports suggests 13th-century destriers were a stocky build, and no more than 15 to 15.2 hands (60 to 62 inches, 152 to 157 cm). Three centuries later, warhorses were not significantly bigger; the Royal Armouries used a 15.2 hands (62 inches, 157 cm) Lithuanian Heavy Draught mare as a model for the statues displaying various 15th- and 16th-century horse armours, as her body shape was an excellent fit.
Although a large horse is not required to carry an armoured knight, it is held by some historians that a large horse was desirable to increase the power of a lance strike. However, practical experiments by re-enactors have suggested that the rider's weight and strength is of more relevance than the size of the mount, and that little of the horse's weight is translated to the lance.
Further evidence for a 14-16 hand (56 to 64 inches (140 to 160 cm)) war horse is that it was a matter of pride to a knight to be able to vault onto his horse in full armour, without touching the stirrup. This arose not from vanity, but necessity: if unhorsed during battle, a knight would remain vulnerable if unable to mount by himself. In reality, of course, a wounded or weary knight might find it difficult, and rely on a vigilant squire to assist him. Incidentally, a knight's armour served in his favour in any fall. With his long hair twisted on his head to form a springy padding under his padded-linen hood, and his helm placed on top, he had head protection not dissimilar to a modern bicycle or equestrian helmet.