Buttercup weighs about 950 lbs now,so she's not so little! Sweet gal, though, and I wouldn't trade her for anything in the world.
I do can milk, even though there are no USDA guidelines for doing such. Jackie Clay of Backwoods Home magazine gave a couple of different methods. I chose to Hot water bath mine for an hour after the water started boiling again. I'm only going to be using it to make yogurt, though, as I don't like the taste of cooked milk. I'm freezing milk for drinking, cereal, etc. for when my cow is dry.
Why would you need a bigger barn for a cow? As long as you have a small place for a milking stanchion and she has a run-in shelter she'll be fine. A three-sided shelter is enough even here in the frozen tundra of Wisconsin. Some cows will even allow people to milk them out in the pasture, but mine is too wiley for that.