Depends on how much you want to fuck it up with stupid, ugly shit like slamming and bagging.

It's a truck. Are you going to use it as a truck, or is your ego all wrapped up in fantasies of a jacked-up purple Day-Glo monster driving down Main Street with tractor tires and air-horns, while wimmen on the street swoon with ecstasy?

16s are good with standard tire profiles. Me, I wouldn't go bigger than 18, just to provide decent suspension movement.