We have 18 stalls in the barn...Show horses at one end and breeding herd at the other. 8'x8' and have never had a problem with that. That said, our stalls are open to the floor on one or two sides and the ones on outside wills all have windows over them. Ventilation was a primary concern for us when we built this barn...and also that the horses could all see other horses to reduce boredom.
There are also 3 8'x16' run in sheds that can be divided into 8'x8' stalls if needed.
We prefer the smaller minis, but have 2 mares that are 33 3/4" and 34" and they foal in the stalls without problems.
I would rather plan a barn than a house!
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Charlotte