I have a 6, 9, and 12" diameter. Since "area" supports whatever is above the post, if building a building, the 12". Fence, just have room for the post and no concrete....in clay. Nothing special about an auger other than keep your rpms slow and keep your hand on your lift, eyeballs on the screw, lift like a saw....up and down, up and down...little bites. When at the bottom of the hole, stop the PTO, lift the auger till the tip clears the ground aand hit the PTO for a second to sling off the fines....reduces what you have to clean out.
I found that my Ford 3000 is my best hole digging tractor with it's Live PTO....just made for fence work. Since I auger slow, HP isn't the big deal and you should be good even in heavy clay like I have. I have augered with my 2000 but it doesn't have the Live PTO and that is what I prefer. If you auger in....auger all the way down and won't come out, get a stilson (pipe) wrench and a long cheater and with slight up pressure on the 3 pt and PTO in N, manually unscrew it......next time take smaller bites and keep your eye on the auger...if it starts moving down fast, as soon as you see it start that, immediately pull up and take smaller bites keeping your eye on the speed of detent.