I originally was sold the recommended 4' sized brush hog when I bought my SCUT which has 19.6 PTO HP ...
Just for fun one day I decided to take the loader off to see what that process was, and then I went to mow ... It was kinda hard to steer in certain situations!
I put the loader back on!
The next year I traded the 4' brush hog in for a 5' version ... My dealer said it should work even though the mower had a 20 HP minimum rating, since I had done all the hard work of clearing the brush, and was only cutting grass 3-4 times a year ... He didn't mention the weight though ...
It weighs 518 Lbs and is 8' long, and hanging off the back of the 69 Lb quick hitch it's quite a long tail out back there ... I find I need some extra weight in the bucket to be able to steer if the tail wheel comes off the ground, or uneven ground!
So it's not just about HP, though I don't think I'd want more than 4' with your tractor ... Which by the way my Bush Hog brand only really cuts 44" and my 5' one cuts 56" ... The entire width of the cutter is the stated width!