@blizzzz1: Yes you need to have the actor in your scene, scaled large enough to cover the surface you want to have reflections on. Alternatively if the object you want reflections for is a blueprint, you can simply add a PlanarReflection component to the BP, and adjust the values there (it should scale to the object size automatically). When you hit Play, the PlanarReflection actor will disappear and leave a reflection on the surface the size of the actor.
@ - Just entered a bug report for the Blueprint based PlanarReflection component causing massive memory/performance issues in editor when adjusting a slider variable value for the blueprint. A video is included as it is difficult to fully explain, along with steps to reproduce in a new project. Thanks.