final RenderBox box = globalKey.currentContext.findRenderObject();

final size = box.size;

final topLeftPosition = box.localToGlobal(Offset.zero);

return topLeftPosition.dy;

class HeightReporter extends StatelessWidget {

  final Widget child;

  HeightReporter({this.child});

  @override

  Widget build(BuildContext context) {

    return new GestureDetector(

      child: child,

      onTap: () {

        print('Height is ${context.size.height}');

      },

    );

  }