none, ideally! although a lot of people say that the mare controls the size of the foal, it's not true, and she will not limit the size of the foetus to what she can easily give birth to. a mare who is an experienced mum is safer to play withthe heights a bit, as well as having some idea of what she 'normally' throws. for instance i know of one mare who always does her foals very very well - she always looks ribby and poor when in foal, and the more she is fed she looks the same but the foals come out bigger. through trial and error her owners have found a regime that stops the foal getting too big in utero, after having a couple get stuck on the way out. so with her i'd definitely stick to a smaller stallion, and had her owners taken the risk on first covering and gone much biger, it could have been tragic.
for a maiden mare who is an 'unknown quantity' breeding-wise, i'd stick as close to the size and build as possible, or go smaller. i'd also research both mare and stallion's pedigree for size info, in case she or he for some reason didn't reach their expectedheight. for instance, a horse out of 16hh parents may have a bad start, or an illness, or just randomly only reach 14hh. but those 16hh genes are lurking so if you put him to a mare of 13hh, which on paper should be fine, those genes may show up and she may have a monster foal.
as an example, petal's mum has had foals ranging from 127cm to 148cm, by the same stallion - so breeding from the 127cm one, you'd need to be aware she has genetic potential to throw bigger regardless of stallion size.