Spark Gap Width

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Chris Fanjoy" <zappyman-at-hotmail-dot-com> 

   Perhaps this is a rather basic question, but I'm hoping someone can 
enlighten this newbie. I've looked around on the web and haven't been able 
to get much help.
   My specific question is, how does one determine what width of spark gap 
is needed? Is there a specific guideline, such as so many mm per kilovolt? 
This is for an air-cooled static gap BTW (actually several gaps in series).
   Thanks for any advice.