BoxCollider defines a rectangular box shape (cuboid) for collision. The size of the box is determined by the combination of the box's Size property and the Transform's Scale .
(NOTE) Recommended Reading: The Collider page should be read before this page.
Size
A BoxCollider exposes the Size property to determine the pre-transform size of this box. This is most commonly done to match the collision to graphics before scale is applied. Additionally, this is sometimes used to set the size of a root object's collision without running into non-uniform scale issues.