The layers library includes three functions for moving and sizing layers:

    LONG MoveLayer( long dummy, struct Layer *layer, long dx, long dy );
    LONG SizeLayer( long dummy, struct Layer *layer, long dx, long dy );
    LONG MoveSizeLayer( struct Layer *layer, long dx, long dy, long dw,
                        long dh);

MoveLayer() moves a layer to a new position relative to its current
position.  SizeLayer() changes the size of a layer by modifying the
coordinates of the lower right corner of the layer.  MoveSizeLayer()
changes both the size and position of a layer in a single call.