be careful on the smaller buckets, it can be much easier to "plug them up" with dirt. having suction holes / water holes built into bucket, can make a large difference.
might suggest a bucket with some teeth on it as well. so you can catch a root on one of the teeth. and pull on up and out. vs root sliding off of bucket.
larger backhoes the physical pins of were bucket attaches, might predict smallest size bucket recommended. other words no real reason to go with a bucket that is smaller width than the connection itself.
for myself i learned there are standardized sizes out there, but due to so many machines out there (backhoes, excavators, etc...) and more so different size machines mini's, to large size industrial units, including age of units. it kinda differ all over the place as far as standards. and in that you might need to use some spacers (basically large washers), or you may need to take bucket down to a local metal shop / welding shop. and get correct holes put in. (cut off / new welded on) to other.
might suggest listing your tractor make/model along with backhoe make/model
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to note it, i have had to dig quite a bit for some larger trees. and i think it is 20" wide bucket, helped a good amount, just wish it was smaller for trenching, and a couple roots, though got away with it, made a bigger mess but got the job done.
trenching bucket, i would of preferred a flat edge bucket, vs a toothed bucket, to make bottoms a little more cleaner, but on other hand, not sure if i could of gotten through the initial hard compacted gravel driveway and a few other spots without a tooth bucket.
