getsize.dart

class MyWidget extends StatefulWidget {
  final GlobalKey myKey = GlobalKey();
  MyWidget();

  @override
  State<StatefulWidget> createState() => MyWidgetState();
}

class MyWidgetState extends State<MyWidget> {
  @override
  void initState() {
    WidgetsBinding.instance.addPostFrameCallback(getSize);
    super.initState();
  }

  void getSize(_) {
    final RenderBox rbox = widget.myKey.currentContext.findRenderObject();
    final size = rbox.size;
  }
  
  @override
  Widget build(BuildContext context) {
    return Container(key: widget.myKey);
  }
}