You can use a cube but I use a stick as it's less intrusive. In Poser I have a stick prop scaled to be about 3m tall. When creating a Vue scene I first import one stick from Poser and scale the scene around this. Then every time I create a Poser figure I add one stick to the pz3 scene from the props library. Then when I import it into Vue, it matches the scale of the scene exactly.