If I'm not mistaken you can buy 4x4 in 6',8' and 10', I think for any bigger than that you have to go an actual lumber yard not one like Home Depot or Lowes.
What Ive found in my own backyard is 4x4s work fine as long as you have 5' in the ground. 6x6 are a bit expensive. The orange sales 4x4x16--tis what I used. I also rented a 12" auger to make getting thru the red clay down here in GA a bit easier. I set the post 18' apart and have 10' of post above ground. Happy hanging!