Building larger version of my PowerSpec Ultra 3D printer?

I already have the PowerSpec Ultra 3D printer for nearly a year. I usually print jobs using PETG. I wish to build larger version of my PowerSpec Ultra 3D printer using aluminum extrusions, longer rods, longer belts, and parts from my 3D printer. The build volume of 9" by 6" by 6" is too small for larger jobs such as the engine block from http://www.thingiverse.com/thing:1911808 I would like build volume of at least 12" by 12" by 12" and I would like to buy OpenBeam or similar aluminum extrusions. I don't know the exact size of the Z-axis threaded rod. Any suggestions?