How good are you?
Myself, I bought a 9" and 14" auger with my PHD. Simply because bigger offers more chance to be off a bit, and still get the posts in line.
The 14" was mainly intended for setting 6x6 posts for my tractor shed. I put 12" diameter concrete stones in the bottoms of the holes to set the posts on, so I needed something larger than 12" so I could be sure to be able to drop those stones in. Fact of the matter is, that the hole always seems larger than the auger...a 12" might have been good enough...
Putting dirt back in is so much easier than taking it out, that I have never minded a larger than necessary hole, when the PHD does the work. Guess I still remember using a digging bar and hand operated PHD too well... /forums/images/graemlins/crazy.gif
I would say if your are going to be putting in 6" posts and want to end up with things in a straight line, get nothing less than a 9" inch auger...