JAR... thanks for that doc! I didn't find my answer in it but it led me to the Volume 2 of ICAO 8168 which goes into detail on the constructing of flight procedures. It basically goes over formulas for calculating the primary area. And based on those formulas it could come up with an infinite number of holding area dimensions. I'm sure they've lowered that number to a more useable one though! I think for the practical pilot it would be good enough to say that if he follows the holding procedures (timing, distance, speed and bank angle) he should be just fine!
Here it is:
http://dcaa.slv.dk:8000/icaodocs/Doc...Procedures.pdf