I used to have a set of 325/65R20's BFG KM2's...
I liked the tires, they didn't make too much noise, got decent traction and whatever. I wasnt impressed with the mileage though. I think I only got about 50 thousand km out of them. A set of toyo MT's or even the Cooper Discoverer STT's I have now go a lot longer for only about 100 bucks more a tire.
The 325/65R20's measure out to about 13 inch wide and 36 inch tall. Nice lookin tires on my truck... I'll see if I can get a pic... But they would rub when I hit big jumps, and that was with a 6" procomp lift, although my tbars are sitting stock height, and even low for stock.
Every truck seems to have slightly different clearances etc.
And for what its worth, I paid 425 a piece for the BFG's. I paid 475 a piece for the Coopers. And the Toyo's are about 500-520 a piece. And that is 35X12.50R20 size. Canadian Dollars.
I will be putting on some Mickey Thompson ATZ's 36X13.50's next time around. Just for the summer though because they are a junk tire, they just look nice. I'll keep whats left of my coopers for the winter haha.