Hugh,
I can't give you a straight answer to your question but sure will throw out an opinion. Kubota list the size implements they recommend for each model in the owner's manual and that would be a place to start. If I remember right Carver Equipment, an advertising dealer on this site, has the Kubota recommendations on their web site. That said, many of us have found Kubota to be rather conservative on the sizes they recommend. I guess that that is understandable when they can't know how hard you will use it or what your soil conditions are like. Florida sand is going to handle a bigger disk than Georgia clay.
As far as the box blade goes, I wouldn't even consider a box that was narrower than my rear tire foot print. If that required smaller bites to match the power and traction available so be it. When you reach the finish grading stage you will want to cover your tracks.
I have never used a disk harrow so I can't offer much about sizes. From what those with experience have said I would look for weight as much as size. I seems that a disk is not very effective without enough weight.
Good luck with your search.
MarkV