How many horses are you designing it for, and does hay and bedding have to be stored in this building too? Do you expect you will close them in to stalls, or do you want them to have cover they can freely access from paddocks?
At the moment, there are 4 and soon to be 5. I don't think we will keep that many, probably 3. I do need room to store bedding and hay, and I want to be able to stall them, but most of the time they will just need shelter.