All the following measurements are O.D.
The bucket that came on my BX23 is 7 3/4" wide at it's lowest (closest to the ground when fully curled) and about 9 1/4" up near the teeth. The point about the clay is very well taken; additionally, the length of the horizontal pin that attaches the bucket links to the crowd is 6 3/4" which would seem to apply a practical width limitation of about 7" to a bucket that is going to dig to any depth. Since direct burial cable (like type UF-B) is usually buried 12"-18" deep, the bucket/crowd links are gonna hit the surface, it would seem, when attempting to dig to that depth. Either that or in loose material you can punch down and ramrod the links into the trench, maybe. Seems like it'd wear the links badly, though. That sounds like a bad idea to me.