The SIZE of an instance as it is defined in a LEF can be obtained using the oaSnapBoundary object which can be obtained from the master of the instance.
oaDesign* master = inst->getMaster(); oaSnapBoundary* boundary = oaSnapBoundary::find(master->getTopBlock()); oaBox box; // <=== four oaPoint objects that describe the bounding box. boundary->getBBox(box);
The coordinate system used in oaSnapBoundary is the block coordinate system, not its parent design.
oaSnapBoundary in documentation,
An oaSnapBoundary must be a rectangle. Placement tools use the oaSnapBoundary to place instances of the design in rows, abutting the oaSnapBoundary edges and aligning the oaSnapBoundary corners. The SnapBoundary will provide an unambiguous lower-left corner that will control the exact placement location.