Suggest you stick with something 48" or smaller. That fits with the idea there should be 5 pto hp for every foot of cutter width. Also, the BX size machines are kind of small and light to be handling a 5 ft. cutter. On any kind of uneven terrain or hills, the weight of a 5 ft. cutter would toss the tractor around.
As for what you might be able to cut with it, I don't know exactly, but the low ground clearance and light weight would make it difficult to drive over anything really big. I have toyed with the idea of getting a small cutter for our BX because it would be handy for places the bigger equipment can't reach, but have always held back because it just doesn't seem to be the right machine, especially for a heavy first-time cutting. If you had someone do that first cut with larger equipment, a BX with cutter would probably be fine for maintaining it thereafter. Having good skid plate protection would still be important.