The county required me to dig the holes for the 6"x6" posts for my project 12" diameter and 56" deep. I put a 2 ft extension on my 12" auger bit and was able to dig the holes with my tractor, but cleaning the dirt out of the holes which are nearly 5 ft deep was another matter!
I found a new hand post hole digger on C.L. for $20 as a parts donor for the digging/lifting blade, and used some 3/4" pipe and other scrap steel I had laying around to TIG weld up the rest. The handles are long enough that I don't have to bend over much to operate them.
You close the two handles together to point the blade straight down, insert into the hole, then pull the short handle down until it locks and pull a scoop of dirt straight out of the hole. Press the small release handle and repeat. I have used it both with dry dirt and muddy mess dirt, and it works pretty well in all cases. Don't know how I would have gotten through the county "hole inspection" without it.