Howald, I have a Yanmar 1401D that is basically equivalent to your B7001, with one more horsepower. I have a loader on mine, and it is 4 wheel drive. I have a 42 inch box blade, and it's just about right. In my soil, a 48 inch blade would be impossible to move once the ripper tines had fully buried. My 42 inch blade will stop the tractor already, but the tractor will drag it easily, full of soil, if the rippers are not buried. I have a loader for extra ballast.
If you are going to to lighter work than deep ripping, the 48 inch should be fine. It will work well for spreading gravel, and let you cover a bit wider area. But if you're planning to rip and move heavy soil deeply, my two cents is that the 48 inch blade will be too big, and will actually slow your production compared to something a size down. Your soil conditions and your goals for it will determine what works best for you. You may consider asking someone with a similarly sized tractor in your area what they have and how it works, or talk to the service guys in a local dealership about what implements they see, and how they work.
