showDialog(
  context: context,
  builder: (_) => new AlertDialog(
  shape: RoundedRectangleBorder(
    borderRadius:
      BorderRadius.all(
        Radius.circular(10.0))),
    content: Builder(
      builder: (context) {
        // Get available height and width of the build area of this widget. Make a choice depending on the size.                              
        var height = MediaQuery.of(context).size.height;
        var width = MediaQuery.of(context).size.width;

        return Container(
          height: height - 400,
          width: width - 400,
        );
      },
    ),
  )
);