I moved into an old property with an old barn that had the original garage on the RH side, and a mirror image door on the LH side. Looks like this~
I have 2 outbuildings, and a 4-car garage. No tractor, yet, but I intend to buy new, if I'm able and I inow where I'll store it. I also had a shelter originally for cattle tacked on the back west side of the barn. It looks like this, and is 16' x 19'. Right now I have 15 chickens in one of the stalls that I created inside, with one horse use the shelter as his stall. HE's pretty cumfy.
To answer your questions:
How big should the stalls be? I am thinking 12x12. They will only be stabled overnight.
Yep. There is room for me to clean around my 16'3hh gelding. He spends the night in his stall from November to April, and on winter days with windy, slushy, snowy and ice weather.
Will a five gallon bucket in each of their stalls suffice for overnight?
Yep. That's what I use. Again, my big guy needs it filled only 2x/day
They have a tank in their day pasture. I store mine during the winter so that I don't have it freeze and crack. I use a 20-gal round rubber watering bowl that I can clean out the ice, if necessary, and carry buckets to it.
How big should the tack and feed room be?
I could use larger, but what I have is adequate. My barn already had two 5' x 10' rooms, and I use one room for grain--both have doors, which you WANT to keep your horses away from your grain. I keep 6 large steel garbage cans which each store about 150 lbs of grain. Also, I have an oil drum sized container and that stores about 250 lbs of grain, AND I made a shelf from 2x4's and 1/2" plywood, 3 ft deep for additional storage. My other room is a tack room. I have a loft for hay, straw, pine shavings (bedding) and Equine Fresh, and I can store up to 500 bales. In MY tack room, I store saddles and bridles, etc. I keep a locker with clean shipping boots, quilts and polos, and an 18gal. caddy with extras of those, plus clean towels, etc. for emergency wound care.
My halters which all have leads attached, are hung along the entry aisleway for easy reach and to easily put them away. I used to use a steel storage rack, but I found a free, wooden large tv cabinet for free on somebody's lawn last summer and I love it. (They waved from the house when we picked it up from their curb.) The cabinet below has 2 doors and each has 2 shelves, there is a shallow shelf above and I also store above the cabinet, itself. I keep all extra basic meds, leather care stuff and extras in caddies. I keep one grooming carry on top.
Will cross ties in the aisle be enough or do I need a separate groom/tack up place?
Separate is really nice, but I make do with tying them in their stalls. That is good training for them. Not fond of cross ties, myself.
I've got many books which show outside storage for sawdust. If I had that I don't know where I'd get the sawdust. You could build one and do that, but you aren't safe using anything but pine shavings. Sawmills and lumber companies cut types of wood that are toxic to horses. I use packages of pine shavings, and I'm comfortable with that.
I have and prefer a cement floor. The first time you get mud between your stalls, you'll regret NOT having one. My aisle isn't large, and I put my rubber trailer mats down THERE so they aren't in the weather, but easy to retrieve. I have 1/2 inch rubber mats inside my stalls--wouldn't do it any other way.
I'm happy with my setup. In my dreamworld, I would also have a nice indoor arena on the east side of my 3-acre pasture. I'd ride there, have stalls there and park my trucks and trailer inside there, too. =D
Watch where you feed your barn cats bc we've gotten both raccoons and possums in the barn seeking cat food. You HAVE to feed your barn cats to give them energy to hunt. I feed the cheapest dry food I can find, but they keep my HOUSE free from mice, and the barn only have nocturnal mice in it.