Under “Rooms” you set maximum width and length in centimetres and wall height: those values power the 3D view and orbital tour. You can upload a floor image and a ceiling image to style materials (wood, concrete, open sky, and so on).
The room editor lets you draw the perimeter on a plan: each edge can be linked to a different wall so the walkthrough matches real architecture or an invented gallery layout.
If you change dimensions, check that assigned walls still make sense with the new outline.
Wireframe: rectangular plan, floor and ceiling texture thumbnails, and an edge linked to a wall.