I have a 5' KK box blade behind a BX22. I can use it to move some of the hard clay I have around here, but once it fills up AND digs, the tractor will sit and spin (but this is somewhat avoidable it you take thin cuts, it just takes longer). If I unload it and go back to the place I left off it's ok again until it fills.
But if I'm just leveling (front blade tilted up to catch the high spots, rear blade to smooth the low spots), it works really well.
Overall, I'd prefer a smaller box blade but like I said, my experience is in some pretty tough clay. Your performance in other soils will vary based on that soil.