positionedtest.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Stack(
        children: [
          Positioned(
            right: 25,
            bottom: 100,
            child: MyWidget(),
         ),
       ],
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return FlutterLogo(
      size: MediaQuery.of(context).size.height * .25,
    );
  }
}

In DartPad: https://dartpad.dev/f94d70e9ccbc4746ded9e7f9d39e4edd?