Should be factory size!
I am about to put my third set of 235/85R on. I tried 255/85R which is even taller and had no problems with rubbing, just could not get much life out of the 255's. Lots of commercial options with the 235's. I have ran two sets of Toyo M55. I can highly recomend these. Very long life even with pulled hard. Not noisy and smooth riding.
I would check you MPH and Odometer with a GPS after new rubber on. Most trucks from factory read a faster speed. Larger tires might put it dead on or very slight off.
jjw
ND