Interior size?

Got a little problem here: I'm working on rendering interior objects in Frank B's MapGui... and so far it works showing a little bitmap dot for each interior... but how do I get the size of the Interior object do draw a rectangle respresenting the object?
I'm using

...
if (shape->getType() & InteriorObjectType) {
   Point3F newCoord = shape->getPosition();
   Box3F shapeBox = shape->getWorldBox();
   F32 objX = shapeBox.len_x();
   F32 objY = shapeBox.len_y();
   ...
   // todo: scale object size down to fit in the mapgui
   dglDrawRect(RectI(newCoord.x,newCoord.y,newCoord.x+objX,newCoord.y+objY),mColor);
}

but I'm not sure that's correct..?!
And I'm not sure how to scale it down to fit the mapgui?
Anyone messed around with Frank's code yet?
Thanks!!