Mares size has a greater impact on the size of the foal then the stallion.
On the ET issue and size of the surrogates whilst the vets say the size of the surrogate mare has no impact on the size of the foal at maturity many of the people paying for the process are not so sure. My warmblood breeding friends, where size matters, are mightily relieved when their embryo arrives home from the clinic in a mare over 16hh.