Montana Farm Acreage Question?

Ok, I'm looking into building a house in Montana (beautiful country out there) and I have no idea how many acres I'm going to need. Let me give you a list of everything we're going to do with the land, a 5-bedroom house (still working on the sq-ft measurements), a big barn (we'll be raising pigs, cows and horses), a duck pond and then of course there needs to be be fenced grazing land. Any suggestions?