Remember that every vet has their own opinion. My sister had two vets give check her mare the year before she was bred. Her vet saw no problem with breeding her maiden mare while the other was convinced that because of her weight (way easy keeper) that she would be killed by the pregnancy or labor. Her colt is almost a month old and she is a wonderful mother.
There is no real evidence of breeding a stallion that is taller than the mare to cause problems. But like any person, they are free to make their own opinions no matter the topic. I have met many people that would never ride a pregnant mare and would advise others to do the same. The size of the stallion does not affect the foal's size at birth, they may sprout much faster after birth though. Even so, you could breed to a 18h stallion and get a foal that matures at 16h or breed to a 16h stallion and get a foal that matures at 17+h. IMO