new coil specs

Hello all, I'm getting the urge to build another coil, which of course
means I have questions!

My major question is what is the relationship of sec. diameter to a)the
spark length and b) the input power needed?  In my case I'm deciding
between 6" and 8" PVC.  Right now I have only one 12/60 NST but plan on
getting more soon.  I have MMC capability between 5 nF and 30 nF.

Finally, I would like to start building a S-RSG.  Does anyone have
step-by-step info on how to do this?  I am a complete newbie when it
comes to this aspect.  My understanding is that you need a special
motor, and that you need to modify it to operate syncronously.  What
kind of special motor, and how to modify it, I am clueless! :(  I've
gathered that this mofication can be very tricky and time consuming.  Is
there a motor that can be purchased to avoid having to do this?

I'm sorry if I'm reasking recent questions!