thanks for the comments guys.
I was going to use 12x12's for the corners and gate. but the biggest bit for the 138 buck digger was 12". I would have had to rent a skid steer at 400 plus per day, plus delivery and pickup, to dig bigger holes. or pay a guy all day to dig 2-3. I may do that next time when this fence wears out. but going bigger, there is also the issue of weight. when using white oak ( all this sawmill cuts is pine and oak logs), it weighs about 80 pounds per cubic foot. the 10 foot 8x8's are 350 pounds. it was all my guys could do to carry one beween two guys. a 12x12x10 weighs about 800 pounds. I am not sure 4 guys with a sling could move those. we would need a flatbed and a off road forklift to play with them.
Cedar weighs about 1/2, probably would have lasted longer, but man, the price. a 8x8x10 cedar post is 250 bucks each. ten times what I paid. cedar 12x12x10 is 460 bucks each. I called a local logger to see if he could find some cedar trees for a fence, and he told me to get on a 2 year waiting list. any cedar tree bigger than 10" in diam. is already spoken for for the next 2 years.
the gate will be 2 10 foot wide swingers. 2x10picture frame, 7 feet tall. in the middle of the picture frame running the 10 foot length will be a 2x6, and all wood unions will have 1/8 steel L corner ot "t" brackets. we will probably cut those ourselves. running top to bottom every 3 inches inside the picture frame will have 1/2 galvanized electrical conduit pipe.
I will weld a 1 1/4" square frame with corner braces and screw it to the back side of each gate to keep it square and support hinge. may bump that to 2" square once I get a feel for the weight.
the posts for the gate actually had deeper holes and the longer 8x8's. we used 3 bags of concrete for those. but maybe not enough. the fact that thre gate is cut in and not along the length of the fence also means it may sag. but I hope not.
I got some hinge brackets from a fence company. they are crazy looking strong, require a 1 1/2" round tubing to act as pins. this will be welded to the frame mentioned above. each hinge, I will use 5/8" bolts, 9 inches long, drilled through the post. so each gate gets 4 of those.