ViewportHeight and ViewportWidth are used to define the view extent when Isometric Drawing Mode is enabled. In Perspective mode the width and height are naturally defined by the camera's field of view, but in Isometric mode the light rays are parallel so additional information is required. If these parameters are set to less than or equal to 0, a width and height will be estimated based on the camera's distance from the surface, which may lead to inconsistent isometric view extents in bookmarks, animations, and exported views.

Reference

Camera Class
Camera Members