I've used a 48" box blade with our BX for more than 10 years. As others have noted, it's when you get it full of material that it gets hard to pull. The 48" size is plenty enough blade for a BX-sized tractor, I think. There wouldn't be enough traction for getting meaningful work done moving/redistributing soil and gravel with a larger blade. If all you are doing is light grading and smoothing of a soft surface, yes you could go larger but it would greatly compromise the ability to do heavier work.