To properly raise a cow and allow it enough room to roam and graze you need to have between 1.5 to 2 acres of land for the cow to be happy and healthy.
Having more acres of land is better if possible but just for one cow it needs at least 1.5 to 2 acres and if you have more than one cow you should have 3 to 9 acres depending on how many cows you have or plan to have.