Container
Let's clear the codebase and remain with an empty Scaffold, then inject a Container widget on the body of the Scaffold.

The widget will have height of 100 by 100 pixels and a background color of blueGrey.

Note: As opposed to the last codebase, there's a minor change. The MaterialApp is now the root widget to allow for MediaQuery sizing to work.
// lib/main.dart
import 'package:flutter/material.dart';

void main() {
  runApp(
      const MaterialApp(
          debugShowCheckedModeBanner: false,
          home: MyApp()
      )
    );
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});
  @override
  Widget build(BuildContext context) {
    return SafeArea(
        child: Scaffold(
          body: Container(
            height: 100,
            width: 100,
            color: Colors.blueGrey,
            child: const Text("Hello World"),
          ),
        ),
      );
  }
}
The text looks a bit small, let's increase its size and make it white and bold for visibility.
// ...
child: const Text(
    style: TextStyle(
        fontSize: 20.0,
        color: Colors.white,
        fontWeight: FontWeight.w700
    ),
"Hello World"),
// ...
For those who don't like to use static size constraints, let's set the height and width dynamically w.r.t the screen size.
body: Container(
   height: MediaQuery.of(context).size.height * 0.3,
   width: MediaQuery.of(context).size.width,
   color: Colors.blueGrey,
// ...
Let's add a margin on the x-axis only and an overall padding for all sides
// ...
body: Container(
    height: MediaQuery.of(context).size.height * 0.3,
    width: MediaQuery.of(context).size.width,
    margin: const EdgeInsets.symmetric(horizontal: 10.0),
    padding: const EdgeInsets.all(20.0),
    color: Colors.blueGrey,
// ...
Then, let's set a border radius all around the container of 10 pixels.

Note: When you introduce BoxDecoration class to the Container widget, the color parameter should now be in the class.
// ...
body: Container(
    height: MediaQuery.of(context).size.height * 0.3,
    width: MediaQuery.of(context).size.width,
    margin: const EdgeInsets.symmetric(horizontal: 10.0),
    padding: const EdgeInsets.all(20.0),
    decoration: const BoxDecoration(
    borderRadius: BorderRadius.all(Radius.circular(10.0)),
    color: Colors.blueGrey,
// ...