WHAT DIAMETER PIPE OR TUBE SHOULD I USE?
It really depends on how big you want your Pyramid to be.
If you want a Copper Pyramid that you can set up in a room and you meditate in then you probably would order US 1/2 inch or 15 mm UK/Euro or 19 mm Australian.
For Aluminium you would use 16mm or US 5/8 inch.
For a Russian Pyramid you would order US 1/2 inch PVC schedule 80 or 21 mm.