LDB, I have a bushhog brand, there is a picture in my garage. This was one of the heaviest duty I found. great for clearing land with the drop down tines. It was a bit pricey @ $ 800 but Im a firm believer you get what you pay for!
Usually you want the box blade to be a little wider than the rear wheels of the tractor. If you get much more than the width of the rear wheels it will be difficult to get enough traction to move dirt or gravel. I probably wouldn't go any larger than a foot wider than the rear wheel width.
I checked the manual (when in doubt RTFM!) It recommends max width of 54". The only box blades I can find are either 48" or 60". I am leaning towards the 60" from King Cutter. Any thoughts?
There are 54" blades out there --- check out Gearmore for one. And, if you want to do anything very serious with the box blade, you'll really appreciate a hydraulic top link.