Caveat that 6" answer. 6" clearance between the closest points between the underside of the 5er overhang and the pickup bed is "normal" for folks that stay on pavement and never cross a dip or ditch, and never go out into the rough stuff to find that perfect boondocks camping spot.
The most I could get with my last rig without drastic measures was 5". But I had to cross ditches and dips and turn around in kinfolks rough pastures, and as a result I had two big depressions in the top of my bed, one on each side. Didn't hurt the trailer, and the bed wasn't ruined, but it was definitely wounded.
So I would say 6" minimum if you promise to stay on pavement and never cross a dip or ditch. Otherwise, 8".
And you cannot simply raise the hitch or lower the pinbox, because the floor of the trailer must be level front to rear after you have adjusted the hitch and or pinbox.
Grumpy ole man with over 50 years towing experience. Now my heaviest trailer is a 7,000-pound enclosed cargo trailer, RV is a 5,600 pound Skyline Nomad Joey 196S, and my tow vehicle is a 2012 F-150 EcoBoost SuperCrew.