MyClass displayObject = new MyClass();
add(displayObject);
setWidgetLeftWidth(displayObject, 0.0, Unit.PCT, 15.0, Unit.PCT);
setWidgetTopHeight(displayObject, 0.0, Unit.PCT, 30.0, Unit.PCT);

public MyClass extends AbsolutePanel implements RequiresResize {

@Override
public void onResize() {
Integer maxWidth = getOffsetWidth();
Integer maxHeight = getOffsetHeight();

// ... need to use maxWidth here...
}
}