Flutter layout

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('Layout Builder Example')),
        body: Center(
          child: Container(
            color: Colors.blue,
            width: 200,
            height: 200,
            child: LayoutBuilder(
              builder: (context, constraints) {
                return Container(
                  width: 300,
                  height: 100,
                  color: Colors.red,
                );
              },
            ),
          ),
        ),
      ),
    );
  }
}