17 inch is still a decent looking size without getting into the cartoon truck look. Sidewalls are still tall enough to look and ride decent. I had them on my 2K cad dts and have them on my 98 BMW 528I.
Tire size wise, this comparison chart lets you compare two tires and you can even change rim sizes by several inches. Plus it is just fun to play with.
https://tiresize.com/comparison/
Searching shows that the C-4 guys can't agree on how to measure from wms to wms let alone get a measurement. You do need the weight of the truck on the rear suspension so that the axle shafts are pretty parallel to the shop floor or sitting how they will be running down the road. You may have to cheat and slide under it and measure from rotor surface to rotar surface and then pull a wheel and measure from that surface to the wheel mount surface and X2 and add that to the inside measurement.
Snooping on Rock Auto the rotors average 2.5 inches tall sitting on the table. Meaning from back surface to wms would add 5 inches to the width if you cheat and measure it that way. Bed is 50 inches plus a few thick frog hairs and outside to outside of the rear fenders is 74-1/2 subtract 1 inch for fender lip on each side and you have 72-1/2 minus what you are comfortable with as far as clearance on the inside and out. My 255 tires are 10 inches wide and that doesn't leave a lot of comfort zone with less than an inch on the inside and not much more on the outside.