Tire upsize

I have a '02 Buick Park Avenue. My question is does anybody know what the largest tires I can put on my car are? I just recently moved down a river road that is gravel and a good majority of it is ROUGH af. I want to get away from 4 ply tires and get some 6 ply and run them a little lower psi to compensate for the lighter load. Any HELPFUL/positive input is appreciated l.