At three years old all my Minis are finished, so anything after that would just be either putting on topline, or "creeping" on maybe half an inch.
I always allow for the difference between American and European horses in height, due to the difference in our measuring, but I will not breed from any mare under 30", which if you put that at 29, possibly 28" American you would be about right. So, I have two fillies I am selling as "not for breeding" right now, one is 28" at three going four, and the other is 27" at two going three, even though both these fillies is fairly well built, I just do not think it is worth the risk. I, too, lost a little mare due to a dystocia, it was just impossible, and that was mainly because there just was no room for manoeuvre in there. This mare had foaled successfully two times before and the foal that killed her was tiny and thin boned, it was just dead, and therefore did not help with the subsequent foaling at all. Sometimes whatever precautions you take, the outcome is dire, and, with a bigger mare, with room inside, I do feel the mare might have been saved. Her cousin, who is three inches bigger, lost her foal due to some flaming gypsies chasing her, but the Vet was able to turn that foal and get it out, and the mare has foaled agian, since. But she is 31", and the amount of elbow room inside, though tight , was acceptable.
It is a judgement call, I feel, people know their own stock and they know what sort of emotional stress they can cope with.
At the end of the day, I say "no, it is too small" someone else will say "yes, it is fine"
So, it just depends on which of us is selling the horse!