Height is multifactorial, so it can seem random at times. It is several genes together so you can have other horses in the pedigree show more similarities in height than the parents. If the population is all similar in height, then you see consistency.
I wouldn’t breed to a pony as you may get a Franken horse, one with characteristics of both breeds, aka big head and little legs. You want to find a talented stallion that tends to throw small. He may or may not be smaller himself. Contender is known to throw small but not consistenly. Few will throw it consistenly. But you also do not know how your mare will throw and it is possible that she will tend to produce much smaller than herself.
If you breed for a really good dressage horse by using a really quality stallion that matches your mare in pedigree and type and corrects her weaknesses, then you should at least end up with a good dressage horse that may or may not be tall. If you breed for height as a main goal, your chances of ending up with a FEI dressage horse will be much less.
Breeding mainly for height may give you an average horse the right size…lots out there already.
Try KWPN as they do meticulous stats on their stallions, and pick an older stallion with a really good track record. You will probably end up using frozen then.