The probe size is only tangentally related.
If you are doing pure galvanic, the probe size wont matter much at all since you are carrying the same amount of current into the surrounding tissues creating the same number of units of lye.The units of lye are computed as timeXintensity and probe size doesnt matter.
When using blend, the probe size comes into play in relation to heat distribution and heating pattern.Since increasing probe size increases the surface area and point effect of the heat pattern.
Seana