如何在Flutter中设置容器内按钮的高度和宽度？

我想在容器内设置宽度和高度固定的按钮。但是，由于某种原因，按钮占用了Container的高度和宽度。我尝试使用SizedBox，ButtonTheme但它们也无法正常工作。

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

void main() {
  debugPaintSizeEnabled = true;
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      // home: EditorScreen(),
      home: Scaffold(
        body: SafeArea(
          child: Container(
            height: 65,
            width: double.infinity,
            color: Colors.black,
            child: SizedBox(
              height: 50,
              width: 50,
              child: ButtonTheme(
                minWidth: 50,
                height: 50,
                child: FlatButton(
                  onPressed: () {},
                  color: Colors.red,
                  child: Text("Test"),
                ),
              ),
            ),
          ),
        ),
      ),
    );
  }
}