Ok, here we go. I found a Perry Agincourt knight, a Fireforge Northman and a Claymore Crecy knight. Didn't have any Fireforge crusader horses on comparable bases, but they are very similar in size to Perry so should be possible to extrapolate.
Perry Agincourt knights:
Fireforge's fantasy Northmen:
Claymore's french knight:
Some quick reflections: Very similar in height to Perry, but a little narrower. Might be tricky to swap riders back and forth, but they would still probably look ok right next to each other. This means they would probably also look good next to Fireforge's older historical ranges. The Fireforge fantasy horse is a little bigger and beefier, but it wouldn't look weird to have heavy cavalry on Fireforge horses and light cavalry on Oathmark horses in the same force. Finally, the metal Claymore horse seems to be a hair smaller.
By the way I reposed some of the Oathmark horses by cutting off the grass supports and giving them a light bend here and there. Not difficult at all, and a good way to vary up the overly dynamic poses.