There's one person with a full Percheron mare at our barn (came from the same foal crop as Aires from the same breeder), but I'm pretty sure they use a full draft bridle with Liberty. The only other horse with a head even close to the same size as Aires' was another boarder's horse, Winston, that sadly died of colic last weekend, but I think Winston used a regular horse sized bridle (he was a belgian/QH cross, but had the more "refined" features of the QH...really just looked like a QH on steroids lol). I know for sure that Aires' head was bigger because I tried Winston's halter on him when Dave got him a new one last month and it was way too small.
I talked to the trainer and asked her opinion. Told her Aires' measurements and the measurements for the draft versus oversized bridles. She recommended going with the draft because there is room to grow. So, draft it is.