Sparkkky, how tall of blades do you want to have? Is your land flat, or rolling hills? My guess is with 22" blades, 10' would be best. Your going to need all the weight you can get. So fill those tires & get those cast iron weights. There are some guys on here that pull 12' disc sets with 50-75hp tractors. I sure would like to know how, my D6 pulls my 12' offset disc in 2nd gear uphill & 3rd gear on the flat & downhill. My 7520 would never pull my 12' disc. Not for lack of power as much as loss of traction. And my tractor is on the heavy side. I have an 8' set of offset disc with 22" notched blades that I use with my 7520 & Case 310 dozer. A guy that I know has a 12' offset disc with 48"blades /forums/images/graemlins/blush.gif, it takes a really big tractor to pull them.
Good luck
Brian