code: def calc_scene_bbox(self):
          
          bbox_min, bbox_max = None, None
          for node in self.root_nodes:
              bbox_min, bbox_max = node.calc_global_bbox(
                  matrix44.create_identity(),
                  bbox_min,
                  bbox_max
              )
  
          self.bbox_min = bbox_min
          self.bbox_max = bbox_max
  
          self.diagonal_size = vector3.length(self.bbox_max - self.bbox_min)
docs: Calculate scene bbox