Transform_scale

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: MyStatefulWidget(),
    );
  }
}

class MyStatefulWidget extends StatefulWidget {
  MyStatefulWidget({Key key}) : super(key: key);

  @override
  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
}

// State

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  Matrix4 matrix = Matrix4.identity();
  double fitWidth;

  @override
  void initState() {
    fitWidth = 0;
    super.initState();
  }

  double screenSizeWidth(BuildContext context) {
    if (fitWidth != 0) {
      return fitWidth;
    } else {
      return MediaQuery.of(context).size.width;
    }
  }

  Widget build(BuildContext context) {
    var sizWWidth = screenSizeWidth(context);
    print(sizWWidth);
    return Scaffold(
        appBar: AppBar(
          title: const Text('Sample Code'),
        ),
        body: Transform.scale(
          scale: 0.5,
          child: Container(
            color: Colors.red,
            width: sizWWidth,
            height: 400,
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            setState(() {
              fitWidth = 800;
            });
            // Add your onPressed code here!
          },
          backgroundColor: Colors.green,
        ));
  }
}