48" would be the max for a light duty. I run a 40" heavy duty Gearmore on my
BX2200. It cuts most anything, 14' weeds, even clusters of 2" poplar sapplings, without slowing. Very dense broom grass is the only thing that lugs it a bit.
I like the Gearmore also because the width of the chassis is the same as the tractor. This way, I can see that if the tractor will fit between 2 trees, then the cutter will fit with the same amount to spare as it trails behind.
Available brush hog widths that will work on a BX are 36", 40", 42" and 48". The Gearmore is the only HD model that I know of. All others are LD. I cannot recall any Medium Duty models in these sizes, though there may be some 48" HD and MD that are slipping my mind right now.
If they existed, I think a 54" light duty, and a 48" medium duty could work on a BX with a 22-23 hp engine, if you were just cutting hay. A BX with 15 or 18 hp engine would probably do best with a 36" LD. A
BX2660 might be able to get by in hay with a 60" LD.
The best of luck in your decision!