Example app using LayoutBuilder to get constraints

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(context) =>
    MaterialApp(
      home: MyHomePage()
    );
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(context) =>
    Scaffold(
      body: LayoutBuilder(
        builder: (context, constraints) => Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.spaceEvenly,
            children: [
              Text(
                "Width: ${constraints.maxWidth}",
                style: Theme.of(context).textTheme.headline4
              ),
              Text(
                "Height: ${constraints.maxHeight}",
                style: Theme.of(context).textTheme.headline4
              )
            ]
         )
       )
     )
  );
}