My understanding is that it is better for a waterford to be a little on the bigger size than a little too small to prevent pinching etc but I would go on how well it fits your horse.
The Waterford mouth should be worn ¼ - ½ longer than any other mouthpiece as the mouthpiece needs to curl slightly around the lips to enable the bit to work to its full potential (except the Waterford Pelham as the mouthpiece is worn the normal length).