The enormous 50cm diameter coil mentioned previously, is of course just a 'proof of concept', used to test the Inductance bridge and prove that that sort of multicore cable can be used to form such a coil. There are of course practical difficulties in actually testing it on the Surf PI circuit.
The 50cm coil was built to a spec of 350uH, but, ive since had information that 250uH should be the ideal target inductance. This apparently is what the original Whites Surfmaster used.
Plugging a few numbers into the online Coil32 multilayer calculator here http://coil32.net/online-calculators/multilayer-coil-calculator.html shows that 250uH should be a reasonably close result if a coil is formed using this cable of 10 to 12 inch diameter
I will measure and cut a length of cable to try out a coil of between 10 -12 inch diameter, perhaps i'll have time to fabricate it over the weekend.
One thing that makes this project hard is the seemingly dispersed nature of the project build instructions! I have yet to find any comprehensive guide saying what the actual target inductance is, how to match the damping resistor to the coil, or how to actually set up the pulse parameters, either by test meter or on a 'scope.