Image is here: https://i.stack.imgur.com/ldYUY.png This example code demonstrate: How to add two rectangles, one is take 60% of the screen while other one take 40%. You can change these according to as you want. And this example explain how to get width of the device screen and how to make your component flexible without fixed values. Also I hav…

container_example.dart

//with dart 2 new and const keywords are optional
void main() => runApp(new MaterialApp(home: new HomePage(),));

class HomePage extends StatelessWidget {

  final text = new Text('Text here', style: new TextStyle(fontSize: 50.0),);

  final margin = const EdgeInsets.only(bottom: 10.0, right: 10.0, left: 10.0);

  final backColor = Colors.lightGreen;

  @override
  Widget build(BuildContext context) {

    var width = MediaQuery.of(context).size.width; // Using this line I got the device screen width

    return new Scaffold(
      body: new SafeArea(//I didnt add appbar. this will add necessary padding for status bar.
        child: new Column(
        children: [
          new Expanded(
            flex: 2,
            child: new Container(
              width: width /1.5, // this will give you flexible width not fixed width
              margin: margin, // variable
              color: backColor,// variable
              child: new Column(
                children: <Widget>[
                  new Expanded(
                    flex: 1,
                    child: new Container(
                      alignment: Alignment.topCenter,
                      child: text, //varaible above
                    ),
                  ),
                  new Expanded(
                    flex: 1,
                    child: new Container(
                      alignment: Alignment.bottomCenter,
                      child: text, //variable above
                    ),
                  ),
                ],
              ),
            ),
          ),
          new Expanded(
            flex: 3,
            child: new Container(
              width: width /1.5, // this will give you flexible width not fixed width
              margin: margin, //variable
              color: backColor,//variable
            ),
          ),
        ],
      ),
      ),
    );
  }
}