It depends on the horse. Those Belgian Draughts look as though they could carry around 20 stone with tack, but I may be totally wrong because I know nothing about the breed. Most draught horses are poor weight carriers for their size (Clydesdales and Shires). A really well put together Irish Draught, however, could probably carry 17 stone. For riding on the majority of large fit sport horses cobs and pleasure horses, then an absolute maximum of 16 stone I would think is about right.