This might be a bit late posting but I have put up few rural fences and was taught by my father-in-law who spent 70 years putting rural fencing up.
No rural fence is put up using concrete, just dirt. Some of these rural fences have been standing for up to 60 years.
Corner posts or gate posts or strainer usually have a stay to take the strain of the wire but the posts depending on the length of the fence run the gate post or corner post can be up to 600mm in dia. remembering a fence run can be up to 2 klms or more with a 14 foot gate hanging off the post.
The hole for the post usually has about 3 to 4 inches clearance around it. The dirt is put into the hole in layers and the most important part of ramming the earth is the bottom six inches and the top six inches. Posts should be 2 feet into the ground.
The problem with round posts is over the years the sap wood rots away quicker than main timber. That is why split posts or sawn posts last much longer.
Remember none of these fences use treated timber, just good old Australian hard wood like Stringy Bark, Red Gum, Yellow Box or Iron Bark.