IronHog has given you the essential trade off for weight and size. I'm not aware of anyone with a CK30 and only a four foot BB though. Five foot would be relatively conservative. Six feet is a stretch but for just moving soil would be fine. Remember you can always raise a few of the rippers to use a wider BB in tough ground.
For what it's worth, I'd go for the five foot version in standard duty from a company like Woods or Landpride. The cost of heavier BBs is worth it only if you know you have lots of heavy work. Otherwise you can still get the work done albeit more slowly with a standard duty model. Hard to imagine a standard duty BB being overpowered by a 30hp tractor.
Been using the box blade many hours here lately to clear off a very rough future home site, fix the father in laws back yard, and cut some skidder ruts out of some food plots.
IIRC our box is a older 5.5 or 6' bush hog brand that they consider "medium" duty........rarely need the scarfers with this nice heavy box with the good cutting blade. It will break traction on a DK 40 pretty easy in heavy ground if you let the cutting edge take a good bite.
A good box blade is kinda the universal tool. I use mine to level ground, do about the same thing as a landscape rake, move yards of dirt, and even windrow brush. Rarely the best choice for any job.... but a skilled operator can do about any job with one.