If anybody ever make a research in this same matter, I will write here my results.
I found in an XIX-th century book the information that you only need a silver plate half the size of that which you plate.
So if you plate a copper plate 13X18 cm a silver anode of 9X13 cm will do. Thickness is important only in terms of how long will it be useful, because the silver plate actually replaces the silver in the electrolyte. It actually “melts” in time, but in a LONG time.
Out of 250g of pure silver I made two anode plates of 10X15 cm, 0,5mm thick. That’s enough thickness for a lifetime I think.
There are several electrolytes commercialized for silver plating, I bought 3 liters with 70 euros per litre.
Works perfect. Actually the quality of the final plate depends on the quality of the surface of the copper. Well polished, buffed, just make a perfect mirror out of copper, follow all the steps recommended by the plating electrolyte manufacturer, and the plating will give you a mirror surface of silver.