27mm OD, 3.5mm wall. Aluminum because of weight, and if installed correctly all the forces are axial along the tube, so it's plenty strong. The thick wall allows cutting a thread directly into the tube without any tube adaptors.
If you made it out of steel, you could go alot thinner on the wall thickness. Maybe 1" OD with .065 wall, but you'd have to weld in tube adaptors for the thread:
http://chassisengineering.com/shopDe...ProductId=1572
Hope that helps,
Lanner