Hey goodoleboy. If I was just sinking barb wire for perimeter fence, I might go with smaller line posts.. but that's just personal choice.
I really need to get hand tamp tool. I have just been using my PHD ends or a shovel end to tamp in the soil.. no matter what you use.. it's slow going.
When i did mine.. I set my corner post 6-8x8, and then just used a line post as an angle brace and using some brace wire.
If I'm foing field 4x4 wire, or 2x4 wire, I put in an "H" brace every hundred feet or so depending on fence height and weight.. lightweight fencing.. I might even go 150' or more for a "H" brace. I make my H-Braces from line posts and brace wire. Then I end with another corner post if the end of the run, or if i change directions. Gate hinge posts I'll use corner posts for, if the gate is a 16' or 20', or perhaps a 5-7x8 for small gates.
On barb wire.. I may not use an H brace unless I'm doing over 300' on a run.. I find that 2 good corners will hold the wire fine as long as you have tposts or line posts.. etc.
Soundguy