I created a small scenario with 3 walls and one directional light.
Pls see: https://playground.babylonjs.com/#TC9IWL#6
I do not get, why there is light on the red wall (wall in the middle). According to the hight of the red wall should be completly covered by the shadow created by wall2.
It signficantly improves, if the light source is moved further away.
This means, it seems that light has a certain “size”, correct?
What is the size? Can it be changed?
This is expected for direction light. See the documentation: Introduction To Lights | Babylon.js Documentation
From the documentation:
The light is emitted from everywhere in the specified direction, and has an infinite range. An example of a directional light is when a distant planet is lit by the apparently parallel lines of light from its sun.
You can check the light frustum in the Inspector