Are you exporting as a 3D model or as a 2D graphic? SketchUp always exports all of a model when you select 3D model. To export only a part of a model, you can cut and paste the desired entities to a new model window and then export the new model. For a 2D graphic export, SketchUp generates essentially a snapshot of the current view. So, anything not visible on the current view will be omitted. But also the graphic will capture the as-displayed view including effects such as zoom, foreshortening and perspective, not true coordinates of points.