How to make your app responsive according to screen size in Flutter

Step 1 : You can use below code to take your screen width & height

    double width = MediaQuery.of(context).size.width;

    double height = MediaQuery.of(context).size.height;

Step 2 : from those variable you can give your widget width and height 

Refer the full code, here container have made according to screen size 

import 'package:flutter/material.dart';

void main() {

  runApp(app());

}

class app extends StatelessWidget {

  @override

  Widget build(BuildContext context) {

    return MaterialApp(

      home: MyHomePage(),

    );

  }

}

class MyHomePage extends StatelessWidget {

  @override

  Widget build(BuildContext context) {

    double widthVar = MediaQuery.of(context).size.width;

    double heightVar = MediaQuery.of(context).size.height;

    return Scaffold(

      appBar: AppBar(

        title: Text('Title A'),

      ),

      body: Center(

        child: Container(

          width: widthVar * 0.40,

          height: heightVar * 0.08,

          color: Colors.blue,

        ),

      ),

    );

  }

}