FittedBox

slide.dart

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

const availableSizes = <Size?>[
  null,
  Size(1920, 1080),
  Size(720, 480),
];

void main() {
  runApp(const Deck());
}

class Deck extends StatefulWidget {
  const Deck();

  @override
  State<Deck> createState() => _DeckState();
}

class _DeckState extends State<Deck> {
  int sizeIndex = 0;

  @override
  Widget build(BuildContext context) {
    final size = availableSizes[sizeIndex];

    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: GestureDetector(
          onTap: () => setState(
              () => sizeIndex = (sizeIndex + 1) % availableSizes.length),
          child: Center(
            child: switch (size) {
              null => Slide(),
              _ => FittedBox(
                  child: SizedBox(
                    width: size.width,
                    height: size.height,
                    child: Slide(),
                  ),
                ),
            },
          ),
        ),
      ),
    );
  }
}

class Slide extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.white,
      child: Center(
        child: Text(
          'Hello, World!',
          style: Theme.of(context).textTheme.headlineMedium?.copyWith(
                color: darkBlue,
              ),
        ),
      ),
    );
  }
}