Mini-cows can be kept on much smaller acreage than full size cows. This is probably the main appeal of having them as pets. Cows are much happier in pairs so ideally having 1.5 - 2 acres for two mini cow would probably be what I'd considered minimum. I'm sure there are situations where a smaller pasture can work if you're willing to keep it clean and supplement the cows food year round.
For small numbers of mini-cows like this, I think there is such a thing as too much acreage too; particularly young calves that may fall prey to predators. In the case of 2 mini cows, 5 acres is a really nice size pasture. It's big enough that they can move around and graze on nice grass without taking it down to the roots, while also being close enough to your home that you can keep an eye on them and they can get comfortable being around you.