I'm probably the weird one on this site, but I look at things a little differently. I have a 54" MMM, but if I were to buy a rear mower, I'd get a 5' if new or a 6' trail behind if I could find a cheap, used one.
Yes it's too big for the BX. But the only time it would be too large if you were cutting weeds 6' tall. If you cut that once, then the next time you used it, the stuff would likely be <12" tall and you could easily cut it with the 6'. To me, going slow on the first trip to do it faster on subsequent trips would be a good tradeoff.
Just my $.02.
Ron