I am by far not an expert, but if I remember correctly from some octaban discussions.. I should be able to help you out a bit. Someone can correct any mistakes that may be in this message.
The thing you have to do for the octabans is make sure that the pipe/tubing you are using has an OUTSIDE diameter of 6, 8, 10 etc.. inches. Most pipes are measures from the inside diameter, and the tube will be off as much as the thickness of the 'shell'.
For octabans length.. I really have no idea.. I would assume somewhere between the 15 - 25' mark depending on the tone you want and the diameter of the drum.
Hope I've helped somewhat