What is the length width and the height of a rectangular prism?

Answer 1

They are the distances from any one vertex to the three adjacent vertices. If you start with the bottom, left, front vertex then:

Length may be to the vertex that is at the bottom, right, front;

Width to the vertex at the bottom, left, back; and

Height to the vertex at the top, left, front.

Answer 2

Whatever you want them to be. LOL