I have a KK 48" tiller on my BX23 and absolutely love it.
For many first pass situations, it's good that the tiller is only 5' wide, but for most 2nd pass or second season situations, a 6' would speed up the operation.
If you take a look at the Implement Limitations section of the BX24 Owner's Manual, it recommends a maximum tiller cutting width of 42" and a maximum tiller weight of 375 lbs.
It is always possible to operate a BX24 beyond those limits; in doing so however, one would normally expect premature wear and repairs.
I also have a Kubota BX24. I am looking at two tillers. Both are sold by Agri Supply and are Caroni brand. One is a 32 inch and the other a 48 inch. The 48 is 6 inches and 27 pounds over the Max size recommended in the Kubota manual for the BX24.
~~~~~~~~~~~~~~~~~