As far as I am aware there is no height restriction on horses in International Dressage. Riders normally chose large horses as they normally display a larger movement which is what is required at that level.
Thanks just wanted to know as I have a 15 hand horse which i'm hoping will go far in the future, and wanted to know if he could compete at in international show.
You need to affiliate and move up the levels OP. When you register with BD, you will get a rule book. Anything you are unsure of tack, attire etc will be in there.
There is they have to be 14.3 plus to compete as a horse. I am sorting at his affiliation now, I only asked as I was curious to know if I would be aloud to go that far on him.