Additional, related question to OP.
Do I need to drill holes for the 3" to 4" posts or just ram them in? The smallest auger bits I can see are 4", so I guess the smaller posts would end up loose!
I use a solid iron bar ~ 5' long and ~1" diameter with a vaguely pointed end to make a pilot hole - throw it into the ground & waggle it and throw again 'til I get it at least 18" in - sometimes have to try a few inched further along the fence line if I hit a really big stone - then stick the post in the hole (generally stands by itself long enough for me to get the rammer over it).
I've only got info of limited relevance to the OP's question though:-When the forestry adjacent to my 'wilderness' was re-fenced the (very) professional fencers asked for access over my land (our shared boundary was the stream); they had about 600m to do, stock fence and top wire, 2/3rds was accessible by 4x4 quadbike - just - for the rest everything had to be lugged by hand.
4 man team. They cut a trail through the wilderness (was basically a steep 7 hectare thicket) down to the stream, cut through shrubs/briars/trees on the line of the new fence, rammed in the posts by hand (even the 8" strainers! - using a massive rammer with all four of them wielding it together), ran the stock fence and wire zinging tight and tidied up after themselves. All in it took them less than 3 days - I was impressed.
I did ask them how much they charged even though I knew i couldn't afford it (as I had about 1000m of stock fence to do) and I seem to remember it was £7.50 / m. But that was over 5 years ago and in west wales. Did it myself in the end - only took me 3 winters

- but most of that time was clearing the old fence and the thickets of gorse, briar and blackthorn - once you get into it the fence goes up quite fast.