As the others say, if it is a 4wd model you will likely need different rims as well as tires to maintain the proper ratio. The front wheels need to spin 2-5% faster than the rear wheels, or you end up being terribly hard on your drive train. You _don't_ want to be hard on your drivetrain.....
If it is 2wd only, then really anything will work, but you'd like it to match the diameter of your current tires as much as possible to keep the speeds correct. I believe you'd need to have the rim width as well to get to this diameter with different style tires?
My Ford/NH manual also lists such info, I'd be surprised if it isn't in your manual as well. That would be the easiest.
--->Paul