Bear in mind that, per FM22-102 "Wall-to-Wall Counselling," that the proper size of dimension lumber to be used with counselling varies according to the size of the user - this allows for maximum impact and control, which makes for more effective couselling.
Being a large individual, the correct size for me is either two and a half feet of 4"x4" mill lumber, or four feet of 2"x4" mill lumber.
Try finding a copy of FM22-102 - if you're lucky, you'll get a copy with the reference charge for dimension lumber vice user size appended (else, I'll have to dig mine up.) It's a multiservice manual - it should have an AFI (Air Force instruction,) NAVMAN (Navy Manual,) and whatever the Marines call theirs number applied as well.