I think you could probably pull a 4' heavy duty or a 5' light/medium duty. I've used a 2wd 29 hp to pull a 6' medium duty. If it doesn't pick your front end up when you pick it up, you could probably go 5', especially if you have 4wd.
5' light duty with a 200+# weight is all my L2500 can handle(26hp). Depending on the soil the disk will bury the tractor. But it's a catch 22, untouched ground and you need a heavy disk to cut. To heavy and it's to much for the tractor. Shipping weight on a disk like mine is 500#, my FM is 525#. With my tractor a 4' is not wider than the rear wheels.
If you can use a 1-2 row subsoiler to break it up then disk. Search for some of my recent posts on my disk, subsoiler and plots. All I need for my plots is some water.