As the title says, I'm interested in raw data on horses in medieval Europe. Specifically, I'm looking for their height, weight and armor (did they have it, what kind of armor, how frequent was it). Preferably, those would be based on physical evidence (skeletons, actual armor) rather than descriptive accounts, but beggars can't be choosers...
I'm already familiar with Museum of London's Medieval horse and its equipment, by the by.
In early Anglo-Saxon England (5-7th century), horses were very small (13 hands, give or take, is common). Think large ponies rather than proper horses. You can find some height data tabulated in the excavation report from West Stow, published here as a free download (see p56). Several dozen horses have been excavated from graves of this period (men were occasionally buried with their horses), and these are comparably small.
Here is an article describing equestrian equipment from this period, by Chris Fern (the current Anglo-Saxon horse gear specialist). Most of the horse trappings seem to have been made for use by fighting men, but so far as I know no armor has ever been found from this time and place.