Feed tack and wash stall take up the space of 2 stalls. The wash rack a min of 8' wide and you need to be more specific on how much room is needed for feed - a dozen bales and a few sacks of grain or a hundred bales? How many saddles need to be hung?
Personally, I'd put the tack room across from the wash bay, not next to it, simply because if you have a leak, or even residual moisture seeping in, it can ruin your tack. (I realize that was just an example, 80acorns)
Also, I would consider dropping a stall or two and leaving an area open for storage, either for hay or various equipment, or whatever else comes up. There is ALWAYS a need for extra storage. Even if you have a loft, having space for a few bales on ground level is handy. But you likely would not need a 12x12 feed room for a barn that size. You could probably get away with an 8x12 with room to spare.
My barn is 50x36, my stalls are 10x12 and I do wish they were 12x12 but I wasn't funding the barn so I had to take what I could get. I have a 10x12 tack area and four stalls on on side and on the other, a 12x20 hay storage, a 10x12 feed and tool room (pitchforks, rakes, trimming equipment, etc) and then either 2 more stalls or more storage for shavings, etc, depending on my current number of horses. I wash mine at a pad outside, so we didn't have to worry about plumbing and drainage in the barn.
Well because doing layouts of barns is more fun than what I should be doing, this would be more towards my preference.
Wash, tack and feed are 8'. You could use the first 12X12 next to the feed for hay storage if you had to, or the 14X12 at the end. I would put the bath, wash and hydrant on 1 end together and feed by an outside door. Of course in my mind, the barn would connect to a giant indoor.
The barn I'm at now has a hopper on the outside w a chute to the inside for grain. The feed mill just drives up and fills the hopper and and they fill a feed cart every feeding. That's a much larger barn though.
I would want the tack room in the middle unless you had a grooming stall. But I wouldn't waste the space in my own private barn.
I'd make the aisle a little narrower and give the extra space to larger stalls. This also makes the wash rack deep enough that you could probably fit the bathroom in comfortably behind it, with the bathroom being long enough to fit a water heater in there as well.
I picture it with large sliding doors at each end of the aisle and runs attached to each of the stalls on the outside.