Dirt Moving minimum auger length?

[pd tractor]

I've used the setup on my tractor before, but the auger (maybe 9"?) Was too long,which made it a hassle.
Even though I only have maybe 20 or posts so to set there will be more...
So I have a good idea of what I am getting into, was more concerned about minimum auger length.
I think I will take the advice and go a little wider on the auger.

Thanks everyone!