Rug sizing

[dodgers89]

I have a 16.3hh horse with very broad shoulders. I got him in the middle of last winter and have only just discovered that two of his rugs are far too small. I can't work out if he would need a 6'6" rug or a 6'9" rug. Any help appreciated!

 

[Peregrine Falcon]

Some rugs can also vary in their cut, just like clothes! What does he measure at? If he's broad I'd be tempted to go up a size.