Don't assume bigger (wider) is better for road grading. I find my 48" is perfect, given my technique: one pass down the middle to knock down the hump, one pass down each tire lane. Rippers partially down on all passes, BB level or slightly canted forward.
[OK, IMO, you then need to follow up with a rear blade to properly grade and crown. Major issue: You can't sidecast with a BB, which means you can't get the gravel on the side of the road back on the road where it belongs. Two passes with an angled rear blade does this nicely. Next two passes are with rear blade, not-angled, with one tire on the furrow you just made with rear blade. One pass up and back and road is perfectly crowned. For extra points, swivel rear blade around and drag for glass-smoothness.]
So, even if I had a 6' BB, I'd still make three initial passes down the road, b/c my first pass has to be dead in the middle, to get the hump. With a 48" BB, three passes = 12', which is plenty for any road I've seen.
Also, in loose dirt my 790 is pretty well maxed-out traction-wise with a full 48" BB. If you're planning on actually moving dirt from point A to B with a BB over any distance, I wouldn't go bigger than 5'.
But, if you have a lot of land you're trying to level or rip, but not moving a ton of dirt, then I'd say go 6'.