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. 

What diameter pipe or tube should you use?