Ron, look for a 600lb or more 60"-66" box. That size and weight will serve you pretty well IMO.Never go below 100lbs per foot of width and 150lbs per foot is actually what you want.
Just my :2cents:
To take this a step further the type of work you'll be doing with the box blade matters too.
If your are just going to maintain a road and move loose dirt then a light duty box in whatever width you want will probably be OK.
If you want to do more demanding work like cutting high spots or pulling scarfers in heavy ground you'll appreciate a narrower, more robust box your tractor can pull without constant loss of traction.
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.
Gentlemen, I thank you for the help. It's good to know what to expect. I don't have the tractor yet and when it's here it will take some time to familiarize with the capabilities. Looking forward to the learning curve. Got a mile of driveway and lots of projects lined up.
I feel lucky to have found this forum.
Ron
Ron, if you have a mile of driveway to maintain, I would seriously be considering a land plane grader blade.