Couple more questions here. Will you have a center aisle in the barn? I want to say that any aisle should be at least 12ft wide for easy use. We drive the tractor and spreader thru there, Vet and Farrier trucks can back in, shut the end doors, letting you close out drafts and bad weather while working on the animals. Important feature to have with our winter weather. Skinny aisles cause lots of problems, add work because you can’t drive thru. Cleaning directly into the spreader saves steps and time. We hitch and drive thru the barn with the 4 of 17h horses and various carriages. You might want to go with 14ft aisle because Drafts are just more massive and so are their vehicles like a hitch wagon. Hay wagons do fit better in the wider aisle too, for unloading or overnight storage when rain is coming. Can’t always unload that day.
Next question is how much time will horses and mules spend stalled? We stall longer daily in winter than in summer. It costs more in bedding to have them inside longer. Sawdust deliveries are closer together in winter. With adding 3 more horses this year I am finding we need deliveries even closer together now. With such big stalls of 16 X 16, bedding is going to add up in costs, even for short stalling times. Also add in time to clean big stalls or pay someone else to clean stalls and runouts so they stay nice. It is amazing how much basic chore time increases with owning multiples of equines! I could be done quick, have time to do many extras when I only owned 3 small horses! We insist that each of our 9 horses is stalled daily, is handled and checked over, has a place to come inside out of our common bad winter weather to dry off and warm up. No run-in sheds here. We think that is a minimal care level, can’t lower the standards any more. Can’t get any more horses (yay) either, stalls are full now. Ha ha
No one here has mentioned using tie/standing stalls as an alternative idea. They are very common for draft animals locally. These drafts get daily turnout so they are free to move around then. We have 7 tie stalls here, used with light horses and really like them. They are a big time and money saver for our horse budget, even cleaned to the floor daily. Bedding is deep enough that horses lay down and don’t get any rubs, have dry coats and dry under their hooves. No hoof problems. Quick to clean after every use, with less bedding needed than box stalls. We find the tie stalls are good training tools for the horses, no ticklish ones as you walk by with hay or grain to feed, no kickers, don’t care about hoses dragging in and out. Horses stand tied anyplace for hours after standing in tie stalls daily. Funny enough they all load easily in skinny (36inch) trailer straight stalls, never have a problem. Slant stalls are too small for our big horses to ride comfortably.
You would need to build tie stalls proportionate to the mules, probably at least 6 1/2ft wide, maybe 14ft long. You want them able to lay down if they want to. Never had any get cast in a tie stall, probably because they get up the same way they laid down, no rolling around like box stalls allow. Our tie stalls have a 4inch lip so they step up. Seems to help prevent hanging rump out in the aisle. Geldings often will back out to pee, then step back in, which helps keep wet on end of stall, not under them. Mares are already peeing at the stall ends in a tie stall.
Anyway questions to think on, alternative ideas to consider in planning.