I made a tack room and a feed room from two existing rooms with doors in my barn. I wish that the tack room was wider, but my grain room is just right. It's ~5 ft wide, 8 ft deep and 9 ft high. The door has a hook that latches onto a staple and there is a piece of wood above it. It has shown itself to be totally horseproof and that's the most important thing that you want in your grain room. I bought brand new large garbage cans with lids to store my grain. Mice cannot eat through this. The first year I owned horses I started with heavy duty plastic and one lid was shortly destroyed by mice. Rats are another story... Funny, I still have the original plastic garbage CAN, circa 1985! If you can let your barn cats have access both rooms to keep down the mice--most mice are nocturnal, so you'll only see their droppings. DON'T use warfin--all of your farm animals can be poisoned by it. Mouse traps still work great, if you must use something.
Make sure that it is lit--we hung flourescent lights in ours--and that you lock up your tack room. You do NOT want anyone at your place to grab your horses, grab some tack and go joy riding.
You do not need anything fancy in your tack room. Two 2 x 4's butted up next to each other from one end to the other, perhaps 3 rows of this will hold at least a dozen saddles. You can get new but damaged carpeting and staple it to the wood to protect your saddles. It's also helpful to have one or two collapseable saddle stands.
Unless you live in a warm climate, a desk is nice, but you won't use it in the winter.
