How to adjust TextField width?

Question

I'm re-writing a simple app which calculates averages. However, I have a hard time with a TextField. I would like it to be exactly as on a first screenshot.

The TextField's width is stretched, I don't want that. I want it to be exactly as it is on the screenshot.

Desired look:

What I have:

Code:

import 'package:flutter/material.dart';
import 'package:easy_localization/easy_localization.dart';

class ArithmeticAverageScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('arithmetic_average_title').tr(),
      ),
      body: Container(
        padding: EdgeInsets.all(16.0),
        child: Column(
          children: <Widget>[
            Card(
              child: Container(
                padding: EdgeInsets.symmetric(vertical: 20.0),
                child: Column(
                  mainAxisSize: MainAxisSize.min,
                  children: <Widget>[
                    ListTile(
                      leading: Icon(Icons.help),
                      title: Text('arithmetic_average_help').tr(),
                      subtitle: Text('arithmetic_average_help_content').tr(),
                    )
                  ],
                ),
              ) 
            ),
            SizedBox(height: 16.0),
            Card(
              child: Container(
                padding: EdgeInsets.symmetric(vertical: 20.0, horizontal: 20.0),
                child: Column(
                  mainAxisSize: MainAxisSize.min,
                  crossAxisAlignment: CrossAxisAlignment.stretch,
                  children: <Widget>[
                    Text('arithmetic_average_your_grades', style: Theme.of(context).textTheme.headline5).tr(),
                    SizedBox(height: 16.0),
                    Text('arithmetic_average_grades_one_at_a_time', style: Theme.of(context).textTheme.headline6,).tr(),
                    SizedBox(height: 16.0),
                    Text('arithmetic_average_grade', style: Theme.of(context).textTheme.subtitle2).tr(),
                    TextField()
                  ],
                ),
              ),
            )
          ],
        ),
      )
    );
  }
}

Answer 1

This will help you make generic width on all devices, so here width * 0.3 means it will take 30% of the screen width

 Container(
          width: MediaQuery.of(context).size.width * 0.3,
          child: TextField(),
        )