I remember reading somewhere that you cannot force a castanea sativa to grow small leaves. I now have a small plant and would like to know if it does work and if so when should I defoliate the plant.
Thanks in advance

EnioS56 wrote: I remember reading somewhere that you cannot force a castanea sativa to grow small leaves. I now have a small plant

It is a small plant so this is not going to work out. Let it grow to a big enough size to match the leaves.
Reduced leaf size is not so much achieved by defoliating, but by creating ramification - and adjusted watering / fertilizing.
Leaf size reduction however is not the first thing you should think of when creating a bonsai.

I gave myself a job and a half years ago, first with a horse chestnut then again a few years later with a walnut. Both are trees with really big leaves. I managed to get second growth of leaves a couple of times with the chestnuts but they didn't look right,the petioles were flat and thin. The walnut I didn't risk as the trunk is now looking great and would rather a large tree in my collection than loose it. One of my chestnuts is still in the big pot but the other I've cut the top off completely to see if I can get better branching from the cut as they do in the wild