What is the biggest prey my cat should be able to hunt?

If I were you, living in a rural location, I'd be more worried about what could take down my cat. They're prey themselves and I personally know two people who've found the half-eaten remains of their cats that were allowed to wander freely. If you really care about it, I wouldn't allow it outside at all. If you insist at letting it roam, expect that it's not going to have a long life.