I find that I can till up virgin pasture in one pass with my KK II 6 foot tiller if I go extremely slow, like 1/2 mph or so. I have to put my tractor in creeper gear to get that slow. If I just use Low range 1st gear it tills ok but doesn't work the soil very deep, maybe 3-4". I think because it is a forward rotation tiller which I got because of all the rocks, I have I have to go extremely slow to work the soil 6" deep or so. In creeper mod it would take a very long time to till 9 acres. I could disk it 10 times in the time it takes to till. I do like the nice level surface that the tiller leaves but I think that going that slow over tills the soil as it is like powder when finished but it sure chops up the grass and weeds. Unless it is virgin pasture type, then I prefer to just go a lot faster and do it twice which give good results also.
I don't think it takes a lot of power to turn a tiller so if buying one, I would limit the size to the weight that the tractor could lift. I think this is going to be your limiting factor. In any case, you need it to be as wide or wider than your tractor outside to outside of tires.