Oh. I wouldn't keep a cow. Just stick with the goats.
Unless you really want a cow, you can find the smallest one and use it. Do you sell your milk, or is it just for you?
How much space do you have? My family runs a working ranch and we have 400+ acres. It depends on how much moisture you've had and how "grazable" the land is.
We give 15 acres of land per cow here, but generally I would say 10 acres per cow.
I use cows for milk and goats for pets :)