28" to the withers.
Had two foals unaided, no problem.]
Third foal killed her, and it was tiny...but dead!
The smallest I will breed now is 30" to the withers, I can see no real advantage to breeding tiny mares, and my two current tinies are not on the breeding list, yet one was out of a 32" mare, and stands 281/2" at three and one is out of a 30" and stands 27" at two years old.
My teeny weeny yearling colt, who weighs in at 26" is out of a 32" mare.
I really do prefer the larger mares, there is absolutely NO guarantee they will not have trouble, all sizes of mares have trouble, but, in a 32" mare there is room inside to do something about the problem, in a 28" mare there is not.
Just my two cents worth.....