the only thing to worry about with a hernia is if the peritinum is open and its actually guts you are feeling and pushing back into the belly. If its guts, you always have to worry about some getting pushed into the hernia and getting closed off or pinched and the guts dying. If its big enough to push your fist into, I don't imagine it will heal on its one, those that usually heal on their own don't open into the belly, just the outside layers.
The surgery is not a big deal, and easy on the horse. I would suggest getting it fixed as soon as possible, one that big can get bigger or like I said, pinch of guts and then you have a huge problem.