How much milk does your family drink in a week now?
A good milking Nigieran can produce around a 1/2 gallon a day if she is not feeding kids too. If you milk share, the kids will get half of that amount.
Usual rule of thumb for space for Nigerians is at least 4'x4' bedding space per animal and 100 sq ft of yard space per animal. Obviously more is better as over crowding can lead to a lot of health issues. So you can fit a dozen does into a 12'x12' shelter, but 8-10 would be healthier especially if it is snowy or wet where you are in the winter and the goats spend a lot of time inside. You will also need space for a couple of kidding stalls that are at least 4'x6'.
My goat shed for the does is 12x24 - half of that is general quarters and the other is divided into two 4x6 kidding stalls and the other 8x12 space is where my milking stanchion and feed is stored. I feed primarily alfalfa pellets so my space requirements for feed storage are fairly minimal. If you can afford something larger then by all means go for it - I would love to gone with a 24x36 pole barn if the budget would have allowed. It's very easy to find yourself with more goats then you planned for I started out with 4 does and got up to 14 before the latest herd reduction. I'm back down to 6 does now, but one is about to kid and I really want to keep a doe out of her....
How much space for a milk stand depends a little bit on your milking style. I prefer to sit on the stand behind the goats to milk. So the space for my milk stand is about 4'x6'.
Bucks will need to be housed separately. You can keep a buck and a wether, or for the same feed keep two bucks. They will do just fine in large dog houses, or you can put up a small divided shed. An 8 x 10 shed would be the minimal size I would go with. That way you could have 5x8 for the bucks and 5x8 for storage. Again if you can afford to go bigger do so - as its hard to stop at just one or two bucks. I have 3 now and would have more if I could... but there is only so much you can fit on 5 acres.