If you are going to do him in the field, sooner is better than later. The drugs we use to drop a horse in the field, compromises their ability to oxygenate their blood and is more dangerous to the horse. The smaller they are, i.e. the less they weigh, the safer the drop in the field. If you are going to a clinic to have him put on inhalent anesthesia, you have more room for error with a larger horse.