Hi Tarquin,
Here are some pipe length starting points based on propeller choice for .60 sized engines. The pipe length is measured from the glo-plug to the first pipe baffle (measured from the front of the pipe).
PROP SIZE. LENGTH
10.5x7.5 - 11x8. 14.25 - 14.5 inches
12x9 - 12x10. 17.0 - 17.5 inches
12x11 - 13x10. 21.25 - 21.75 inches
These have worked very well for me for the past 34 years or so. I prefer to keep the pipe length a bit longer, especially for the larger prop loads.
Good luck - what engine, pipe, prop combination are you researching?
regards,
Will