this is probably a minor point depending on what the interior structure is, but if you make 12 foot increments, its easier for plywood and other lumber so you won't have so many shorts left over. 10 foot lumber is not common out here. if you do 12 foot center to center, there is a few inches left over after you trim, especially if you use 6x6 posts. plywood is obviously in 4 foot increments. you could use a four footer and split it obviously. the "standard" for horses is 12x12 stalls. also, 10x10 is not very big for hay. also on the door, consider what height is convenient to move hay into the barn. you probably won't move a stack of 96, but they are pretty common out here. my neighbor has a barn with the hay section so she can put three stacks of 96. i move hay way too much and i'm not that much stronger than dl. whatever size you make, make it expandable. also, install a mister and a room with a low ceiling for a cold room, vents on top,(hot air rises).