Question Avoid going through the ground at any time

Hello,
here a gif to explain my problem (please click on image attached to animate)


My character in blue going through the floor because it is hitting a sphere with sphere collider.
My character have a box collider.
I need to keep thoses collider's shapes.
I can't scale height of the gray sphere collider because my character will sometimes scale down (and could going under the gray sphere).
Finally my floor in red is a cube with box collider.

How can I prevent my character going through the floor like this, please ?