I am currently winding a 6 Inch Minimum, Ruhmkorff Coil. I have a core 18"x1-1/2" I.D. I will wind 17 " of number 12, 2 layers, 250 Turns total. I will have the ability to run with the 250 turns, or 175, in parallel. The outside diameter of the insulated primary will be 2-1/2", so my "Pies"-"Yo-Yo's" will be 6.25", I will probably go to 6.5". So the Sides of the "pies" will be 7"-7-1/2". The Secondary will be 8.75", which is really .75 over specification, however, My inner and outer insulation will be "bullet proof". Also a good coil, should be able to be disassembled. Don't try and max out your spark, it WILL lead to a Failure. If your building a 6" Coil, Don't go over 7". Size 34 wire is good, 36, cannot alway handle the pressure, and it is MUCH harder to work with. Is it that important to have a smaller coil, give a bigger spark? If your in some kind of competition, go for #38 or #40. Have FUN! I could go on with specs, any questions please ask.