What size breast collar?

Horse size is too big and just hangs on her. She's 820 pounds, 14.3 hands, arab. Her cinch is a 30 and its a bit too big too, I'm going to have to punch an extra hole on the strap on the right side. A regular horse headstall fits her fine if it's on the smallest adjustments. Pony sized headstall seems a little small so I was leary of getting the pony sized breast collar? What do you think? I know headstall doesn't have to do with breast collar size but just giving you all the info!