For most internal uses. You don't want to go over 20 ppm as the particles will begin to stick together.
If you don't have a current limiting circuit you probably don't want to go higher than 5 ppm. Current will go up with the conductivity of increasing levels of silver.
Larger electrodes will allow more current without the larger particles. The same amount of current will be spread over a larger area.
Slow stirring keeps the silver from concentrating around the electrodes.