Its a question that a Set Designer sometimes needs to address. Sometimes the PD doesn't know that the height of the Scenery becomes a money decision, not a design choice.
When drafting a Facade of a building, for example, this is usually a non-issue. The building is tall, the Actors safely in front of the Scenery. The trouble arises with a scene like an Arctic Ice Floe, or a rocky hill.
When the film goes into Post and the Vfx Artists remove the green (or blue) to replace it with a background, it is very easy when the greenscreen butts up to the static scenery. When an Actor is in front of the greenscreen, each frame must be rotoscoped around the changing actions of the Actor. This adds a huge amount of extra work in Post.
When you are drafting BG scenery for the greenscreen, find out what the shot will be. If the scenery needs to be taller than the Actor, its really important to do a shoot-off calculation based on the camera placement and the floorplan, to ensure the scenery is tall enough.
Sometimes it can be a bit shocking to find out your BG Ice Floes need to be 8-10 feet tall.