Our barn is 36'x72'. One 36'x36' half is post & beam construction setup for three 12'x12' stalls on each side of a 12'x36' aisle down the middle. As it turned out we built 4 stalls, two on each side, a tack room in one of the corner 12'x12' areas, and then used another corner 12'x12' area for an entry door, water hydrant, fuse box, and a staircase up to a loft.
We built a hay loft over the middle aisle. We used doubled 2x12's for the headers (along the side of the aisle), each spanning 12' from posts at the corner of each stall (actually we added smaller posts 4' away for the other side of the. We used 2x12s 16" OC that span 12' across the width of the aisle. The whole thing is very sturdy and works well. These trusses were covered with 3/4" plywood (no rot/moisture problems since hay is dry, and easy to sweep out old hay before new arrives). We load hay through a hay loft door.
By the way, the other half is built with trusses 9' OC spanning the entire width (36'x36') to keep it open for machine storage. There is a full wall separating the two halves of the barn. There is a 12' wide sliding door front, middle, and back of the barn.