Welcome to TBN. You'll be able to do some light duty work with a 5ft box behind a BX, but it will be limited because you can fill it with more material than the tractor can effectively pull. You would likely get more real work done with a 4ft because you can actually dig, fill and pull the attachment with a BX-size machine.
If you already have the 5ft box, go ahead and try it, but if you're trying to decide what to buy, definitely go for a 4ft size. The 4 ft size is also a perfect rear ballast for loader work on a BX, and, since it doesn't jut out beyond the width of the tractor so much, it is not as likely to snag something.