a pancake 3/8 tube with like spacing and a 1.5" arc space? The secondary is a 8.25" 35.5" 22AWG. I seem to remember Richard Quick or some one post that the aspect ratio to fully envelope the coil should be around 2/3? In this case it comes out to ~ 26" but is taped at 10.85 with a 40NF Beefy MMC cap using Java Tesla.
Jim,
the primary doesn't really envelope the coil, rather it couples mostly into the bottom of the secondary. So it doesn't matter if the primary is wide or not. Wideness will only affect the coupling. You do want the coupling to be as tight as possible without getting racing sparks or flashovers. The reason for using 3/8th inch tubing is to handle the high currents without too much loss. I would imaging that at such power levels as you are proposing, you would want to use 3/8" tubing. I'm not sure how much difference in performance you'll actually see. Richard Hull ran his Nemesis coil up to 11kW or more and used 5/8" primary tubing.