I used 4" for my tool tube. Here's the instructions I wrote up for it.
Quick instructions for you on how I made mine
1 - 24" piece of 4" I.D. ABS sewer pipe
2 - 4" Test Plug adapters
2 - 4" Test plugs
1 - bottle of ABS glue
1 - Hacksaw/chopsaw or similar cutting utensil
4 - 6" hose clamps
1 - measuring tape
1 - white grease pencil
1 - file
Use measuring tape and grease pencil to mark pipe for appropriate length (I made mine 14")
-- remember that the test plug adapters are going to add a couple inches of length to the tube on both ends --
Cut pipe at marked location and clean up burrs from the cut end.
Apply ABS glue to inside of test plug adapter and push on to pipe with a twisting motion until it is fully seated.
Repeat Step 3 for other end
Loop Hose clamps through holes on front of aftermarket aluminum skidplate and tighten them up a little bit (just enough to allow the tube to fit through the clamps)
Hold the tube so that it sits on top of the bolts securing the front of the skidplate and tighten down the hoseclamps, alternating sides so that the tube is held level and secure.
Clean up the mess you made, then sit back, crack a cold one, and admire the fact that you are slightly more ept than Tim the Toolman Taylor.
If you did it right, it should look something like this:
If you are still running the stock skidplate, you might have to devise another means of securing the tube.
If you have the HT pumpguard/highway pegs, you can secure the tube to that as well.