For the bolts that hold the brackets to the frame, I used 1/2" iirc, one that goes through something else was 3/8". I also used 1/2" for the bolts that hold the bar to the links, and the links to the axle beam. The new bolts will have a 3/4" head, the stock ones are a fancy metric size, they have an 18mm head. Some will say use fine thread because they are stronger, but they are sold individually at tractor supply (instead of by the pound) so they are crazy expensive. Use lock washers or nuts, you won't have any problems. I find it hard to believe that the bolts wore out, it sounds like your bushings are shot, but chances are to get the bushings out, you'll end up needing to replace bolts.