That would depend on the size of the miniature horses you are going after. If they are A, R or B grade those are all deferent nights. But the best rule of thumb is to take the length of the horse, as if they were laying down on their side and stretched their neck out all the way and their back legs, and add three feet to that. That should be a good length in depth and length . This will allow for the horse to get up and down, stretch out, and maneuver a baby around to give birth. The extra three feet is for her and for you if she needs help having a foal. Also giving enough room for both mother and baby to get away from each other and for the mother to get up and down without worry of mashing or stepping on a baby.
If you don't intend to breed I would still add the extra three feet. Mini's get sick and you may have a vet out from time to time that will need some of the room. Take the length that I recommended and put that leant in the middle of the stall and then crouch or sit at both ends.
