Go extra large. I have a 16.2 tb ( 6ft3 rug) with a long gangly neck and she has the large. Dont get it too tight becasue when they put their heads down to graze they will get tight around the neck.
I have to say i really really rate the amigos for style quailty and value for money.
This is my mare wearing hers this will give you an idea of the size