There is so much that depends on your soil conditions and ultimate goal that it is really hard, I think, to give advice confidently.
I have an old, heavy International 464 with 44 PTO horsepower and big (14.9x28) tires filled with water. I have a 5 1/2 foot cut offset disk that will stop that tractor in its tracks here in my field. Bulldog's L3000, with well under half my weight, easily pulls a disk almost twice as wide through his. Neither of us are wrong, but different conditions can clearly cause dramatically different outcomes. What works for him would be hopeless for me, and my much narrower unit would be inefficient for him.
If your soil is at all clay, or dry, or is overgrown, I would buy the narrowest, heaviest disk you can find. Weight per blade is the key factor in making a disk cut in unturned soil, in my experience. A compact tractor may be able to easily pull a 6 foot tandem disk, but cannot lift it when enough weight is piled on it to cut deeply. If it cuts deep enough to break up the ground, often the tractor cannot pull it.
If you have the ability to use more than one implement, or do it in more than one trip, then your options expand. A middle buster, or even a box blade with the rippers down, does a fine job opening the ground, and a disk doesn't need to be as heavy to work well in the the opened ground.
If you have loose, fluffy soil, then buy a 5 or 6 foot unit and use it happily. Otherwise I would be looking for something in the 3 1/2 to 4 foot range, and keep on the lookout for suitable kinds of ballast. I have a 42 inch cut offset three point disc I use with about 300 lbs of ballast. It has 7 18" blades per axle and does a reasonably good job. If I were cutting in a food plot with my 22 PTO horsepower tractor, that is the disc I would use. My 6 foot tandem disk takes too much ballast to cut well, so I use it as a finishing disk to smooth everything out once it is initially done by the plow, offset disk, etc.
I would rather make twice as many passes but go over everything once and have it be "good enough," rather than making fewer initial passes but having to do it twice in order to be equivalently ready for seed. It's simply a preference, and I really don't see any meaningful difference except one's own concept of what is optimal.
Will you be needing to mow before you disc, too, or anything like that? Do you have any implements currently?
