Wow, good question. No idea. Seems like there is great variation anyway. Depending on brand, my TB's can wear horse, cob, pony, or yearling stuff. Yes they have little heads compared to the average TB. I'd try calling someplace that sells Carlyle (I think State Line does) and asking if they could measure a cob bridle and a horse bridle. Or maybe you could contact the manufacturer directly.
Alternatively if you're just looking to use it, not sell it, you could just see whose head it fits!