Dogs like this require less exercise than probably any other type of dog. A couple of acres will do them fine. 
However, for other dog types I think it's a mistake to think a big yard is enough. Dogs need to get out and smell things and also need stimulation to get the exercise they need. Even if you have 5 acres you should take your dog for walks and drives to see different places. If you have over 20 acres you can probably get away with no walks but still should get out on your property making your dogs run by riding around on a 4 wheeler or something.

However, yeah, LGDs are pretty unique in how much they are homebodies, they're the one dog you can get away with sitting out back on a couple of acres and leaving them to it.