Monoslopes are usually between 32-35 square feet and slatted confinements are down to 25-23 square feet. Outside lots however I don't have a number for you, I forget that one.
Might not hurt to check with someone at the Cattleman's Association on this. They may be able to help out. I'd say check with the DNR but no need to involve them until it's absolutely necessary. But I'm sure there might be a permitting process involved if you are thinking about construction.