change font size in flutter

                                    child: Text(
	'Hello World! This is a text widget.',
    style: TextStyle(fontSize: 25),
)

                                    /// Android has different 'textscalefactor' settings, like:
  /// 'Small: 0.85', 'Default: 1.0', 'Large: 1.15', 'Largest: 1.3'
/// To make sure the text in your app stays as you intended it to,
  /// use the MediaQuery Widget to overwrite the system default:
@override
  Widget build(BuildContext context) {
    return MediaQuery(
      data: MediaQuery.of(context).copyWith(textScaleFactor: 1.15), // Large
      child: Scaffold(),
    ),
  };      

                                    new MaterialButton(
  height: 140.0,
  minWidth: double.infinity,
  color: Theme.of(context).primaryColor,
  textColor: Colors.white,
  child: new Text(
    "material button",
    style: new TextStyle(
      fontSize: 20.0,
      color: Colors.yellow,
    ),
  ),
  onPressed: () => {},
  splashColor: Colors.redAccent,
);