That is impressive. Generally, arcing can occur at about 10 kV/in, so you have at least 3 kV. Do you have a link to that module? Some of them caution to have a gap no larger than a certain distance, or internal damage can occur.
Here is a similar module rated for 400 kV with 3.7 to 6 volts input, and it cautions against a spark gap greater than 1.5 cm (0.6"). The sharper the electrodes, the lower voltage is needed to ionize the air and cause an arc. That's why high voltage equipment often has spherical or toroidal electrodes, and it's why lightning rods have sharp pointy features to attract lightning and cause it to discharge harmlessly through the ground wire on buildings.