1 1/2 inches, sparks will jump between the two spheres almost as soon as we start to turn Dirod. The breakdown voltage for air is 30,000 volts per linear centimeter of path. Thus, to generate a spark of 1 1/2 inch length takes an initial voltage of over 100,000 volts. Be cautious about drawing sparks with your body - for they will sting. However, the current they carry is very low and not harmful. However, I would be careful not to let small children draw sparks from Dirod.
When a spark jumps, especially a bright one, it momentarily discharges Dirod. However, the charge on Dirod rapidly builds up, only to initiate the next spark. What happens if we separate the spheres beyond the maximum distance a spark will jump, for the voltage we can generate with Dirod. Dirod will still sound very noisy, as coronal discharge takes place at various places, such as between the rods on the rotor. It is fun to run Dirod in the dark, and actually observe all the different sources of the coronal discharges and sparks. Many of the larger sparks between the spheres will be anything but straight, just as lightning strokes tend to have lots of wiggles.
If the quarter is located closer to one sphere than the other, the larger gap may still be too wide for a spark to jump. If this is the case, I have observed considerable corona discharge from the quarter into the larger gap. Again, it is fun to try the probe in the dark, and study the sparks and coronal discharges.
I also observed that for any of the motors I have built, a strong coronal discharge takes place as soon as the motor self-starts. I can't say for sure whether the coronal discharge starts before the rotation starts. Some times a spark will jump between the stator and the rotor. It seems like the motor will always start after such a spark, and then the coronal discharge continues as long as the motor is running. Again, rather than relying on the ear to detect the coronal discharge, it is much more straight forward to run the motors in the dark and observe the coronal discharge.