Distilled water is a fairly good insulator. With a constant voltage connected to silver electrodes in distilled water, the initial current flow is very low because the water has high resistance to electron flow. However, as the silver particles finally do start to accumulate in the water, the resistance in the water goes down from the conductive silver content now building up. As more silver is torn from the electrodes, the process is accelerated until one has to stop the process because the particles become so large they begin to fall out of suspension. If the particles of silver are truly colloidal, they will be suspended in the water because they are not heavy enough to sink to the bottom of the vessel and they are electrically charged and repel each other.
Larger particles are too heavy to remain suspended, so they fall to the bottom of the vessel.
So, uniformity of size is VERY important. That is, only so long as the current flow is LOW . That is why some manufacturers tell you to put salt in the water at the start and run the unit for a few minutes only. Adding salt to the water increases the conductivity of the water which gets the reaction started quickly. You are then told to run the unit for only a short period of time because they know the reaction will be too violent if left to run very long.