We make ours either 12’ x 27’ or 30’.
What determines the size is the size of the metal sheets first.
Then that 12’ is the widest we can haul on a flatbed trailer down the highway if we want to move some.
12’ is deep enough for horses to get out of the weather in there, but not so deep to pin one far down in there that can’t get out.
Also, deeper will leave the back part more musty and moldy and wet, the sun won’t get all the way back there as easily.
If we want deeper, we can add a hinged type overhang we can, once in place, attach back up, to give more protection.
The length depends on the sheet metal sizes and we have made a few shorter ones for a neighbor’s goats, etc.
Longer than 30’ is harder to cross tie down to haul without needing to make the frame stronger.
The number of horses depends on how they get along.
Some horses will fight everyone and be the only ones under there, others may pack in there like sardines and all happy.