I have one for a hanoverian cross. The normal one was a definite no go but XL fitted just fine but isnt roomy. If your horse is really big chested/big shouldered the XL probably wont fit though. The premier equine ones seem to be the most generous
He's a HW cob, he used to have a 6ft3 amigo with a chest extender but he killed it. He's currenty using a 6ft6 amigo that belonged to my deceased ex racer, it fits ok around the neck but just seems a bit long in the back on him and a 6ft6 weatherbeeta that beloned to my deceased idxtb that looked like it fit him well but it rubbed his shoulders to pieces.
I don't want to spend a fortune on a rug as he will only be wearing it in really bad weather or if he drops his weight a little (he's 20 and will be living out 24/7).