Not much flat land for a 36' by 60' barn. The terrain slopes down on all sides.

Anything larger would be more expensive, take longer to build and take up more space that we didn't have. Anything small would be less cost effective. We figured on how many horses a full time Mom could manage with two young boys tugging at her heels. There would have to be enough stalls to rent out to recuperate the costs of building the barn, but not so many that the work was unmanageable.
Once we figured on size then it was time to learn about materials and the process. Fortunately, I'm a Trooper and I fly the state police helicopter. That means I drive and fly a lot, so I see lots of barns.