Hi Yasser. Thanks for sharing this!
The Displacement has a Vector input and you should use a Vector> Displacement Node to connect it, something like this:

No Math Node needed anymore (use the Scale in the Displacement Node).
The Units have nothing to do with the strength of the effect, but the Scale of your Object (in this case the Plane) does!
Scaling up a plane in Object Mode gives large displacement, if you then Apply the Scale, the effect is smaller.
Here’s some more info on the Voronoi: https://blog.cgcookie.com/forums/topic/uvs-texture-colour-map-still-required/