Jon, I believe your owners manual says to use a maximum 42" cutter not exceeding 300 lbs. While there are a few 42" cutters on the market, none to my knowledge is less than 300 lbs. The unavailabilty of cutters for my needs was one of the reasons I traded in my BX for a larger tractor.
Last year my dealer was selling the no longer available light duty Woods 48" cutter for the BX. But because of the BX's tight 3ph geometry, he had to cut special holes in the cutter's A frame to fit it on the BX. Now he is selling a new 42" cutter by Landpride, which looks quite nice.
I think the answer to you question is that a 42" would be the best match, but you may be able to hydraulically handle a light duty 48" cutter if it can physically fit the BX. Dont buy one unless the dealer proves and demonstrates to you it fits and can be raised to proper heights.