Good advice has been given to get a tiller as wide (just a bit wider is better) than the width of your rear tread (outside to outside of your tires).
I have a YM2002D, my tiller is roughly 4.5 feet wide. It is about 2” wider than my outside to outside rear tread width. I have never had an issue with it not tilling soil (including till soil that had not been tilled in about 30 years). I did make several passes from differing directions. This soil has some gray clay (call gumbo in the area) and can cake and bake very hard.
with 3HP more at the PTO, it is my opinion that a 5 foot tiller would be no problem for your tractor. Tread through till soil looks ugly, but is also compacted, it will cause issues for seed to evenly take root.
(my $0.02 worth)
Good luck in your search for a tiller!