Have been shopping myself for a BH. They tell me buyers typically get one mid-sized, all-purpose bucket, say 14-18 inches for a 9 ft. hoe and maybe a bit smaller for the 6.5 or 7.5 ft. hoes. Obviously, the more powerful the hoe, the better it can handle a larger bucket. A large bucket, say 24" or more, can get expensive.
Also, your local digging conditions are a factor. Texas has some tough, rocky soil in many places, where a smaller bucket would have some advantages.