Example: One way to make a dialog bigger

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Alert',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyView(),
    );
  }
}

class MyView extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Sized Alert Dialog'),
      ),
      body: Center(
        child: ElevatedButton(
          child: Text('Alert Dialog'),
          onPressed: () {
            _showDialog(context);
          },
        ),
      ),
    );
  }
}

void _showDialog(BuildContext context) {
  showDialog(
    context: context,
    builder: (BuildContext context) {
      return AlertDialog(
        title: Text("Alert!!"),
        content: ConstrainedBox(
          constraints: const BoxConstraints(
              minHeight: 400, minWidth: 400, maxWidth: 700),
          child: Text("I am biger than normal"),
        ),
        actions: <Widget>[
          TextButton(
            child: new Text("OK"),
            onPressed: () {
              Navigator.of(context).pop();
            },
          ),
        ],
      );
    },
  );
}