The use of a box blade is quite important in looking at its size. If you're going to be moving relatively dry earth around on a relatively flat surface, your MF 65 should be able to pull a pretty wide BB. However, if you use it on gravel on slopes, your tractor probably should look at 6 foot max. My old JD 2030 with probably about the same weight as your tractor, but with 68 hp or about double yours, struggled to pull a 6 foot BB on a fairly steep gravel drive. While it handled it easily on the flat, the hill would sometimes just stop it dead. I downsized to a lighter JD CUT with 38 hp but less weight than yours, and could not pull the 6 foot BB on any hill... Although it did manage flats. I actually downsized the BB to 5 ft and it now handles is pretty well. So the bottom line is the use and slope for your BB. If you buy new, a dealer may let you try it out and go for a different size if it can't pull it. If not, I'd suggest getting a 6 foot max and seeing how it goes. You can usually find someone on CraigsList to swap with to up or down in size.
Some folks like the have the BB wide enough to cover your tracks. Again it depends on your use. With gravel drive maintenance, that really doesn't matter. Just being able to pull it smoothly does.
But, again, if you are just moving dry soil around you can shoot for a bigger blade.