The Over voltage spark gap is essentially just two electrodes with a gap between. When the voltage between the two electrodes exceeds the breakdown voltage of the gas, the device arcs over and a current is very rapidly established. The voltage at which arcing occurs in these devices is given by the Dynamic Breakdown Voltage, which is the voltage at which the device will breakdown for a fast rising impulse voltage. Note that this voltage may be as much as 1.5 times greater than the static breakdown voltage (breakdown voltage for a slowly rising voltage.) how much greater than the static breakdown voltage the actual breakdown voltage is will be depends almost entirely on how rapidly the voltage rise, a shorter rise time means a higher breakdown voltage. Commutation times for these devices are exceptionally low (sometimes less than 1nanosecond).
Overvoltage gaps are primarily used for protection. But in combination with the other devices mentioned here they are commonly used to sharpen the output pulses (decrease the rise times) of very high current pulses form triggered switching devices e.g. Thyratrons.
The size of these devices is almost entirely dependent upon how much current/voltage they are intended to switch, There is really no limit as to the size of these devices they can be as small as krytrons, however they can also be very big, and devices intended to switch MA will be just that.