While I am no expert, I made my wall panels 2in. and my bass traps were 4in.. This was done because of advise I got from here. I have not made any for the ceiling yet, but I think I'm going to make those 4in.. Mostly because I am sick to death of all the stapeling and I have about 24 panels of 2in. left that need to be used up. I've heard of guys using 6in. on the ceiling above the listening position, but I don't know if I will do that or not. I think the big thing is make sure you have an air gap behind the wall panels so the inso. is not tight against the wall.
Big Tom