The best size posts to use are around 9 feet long. You then bury three feet of post into the ground and have 6 feet of post to train with above ground.
Untreated wood can be hard to find.
Many people use pvc posts and fill them with concrete.
My teacher, Shifu John Painter has a Nine Palace Post area with concrete filled pvc posts. They are very hard, and most excellent to train with.
Unique Publications is going to be releasing a new DVD this year about Multiple Opponent Combat using Baguazhang. In that DVD you will be able to see yours truly walking, hitting, wedging through the forest of posts.
Let me know if you have any other questions.
Mouth Boxers have not the testicular nor the spinal fortitude to be known.
Hence they hide rather than be known as adults.