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.