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.