Depends on what you're using the breastplate for?
I'm assuming it's because your saddle slips back? For this, you could also try putting a Chamois under the saddle.
One of those cheap ones you can buy in the store, usually found in the car section (Here at least). Dampen it and ring it out really well, then lay it directly onto the horse's back, put your numnah and saddle ontop in the correct place. This should stop the saddle from slipping backwards or forwards.
That's if you can't find a breastplate to fit. If your horse is very narrow through the chest a shoulders, a cob-size might fit him alright, otherwise, yes, a full.