官方SizedBox documentation：

如果给定一个子对象，此小部件会强制它具有特定的宽度和/或高度。如果此小部件的父级不允许这些值，则将忽略它们。

例如，如果父对象是屏幕（强制子对象与父对象大小相同）或另一个SizedBox（强制其子对象具有特定宽度和/或高度），则会发生这种情况。这可以通过将子SizedBox Package 在一个小部件中来解决，该小部件允许它的大小不超过父级的大小，例如Center或Align。
因此必须删除sizedBox或将ConstrainedBox居中。

import 'package:flutter/material.dart';
import 'dart:math' as math;

void main() => runApp(MaterialApp(
      home: TestConstrainedBox(),
    ));

class TestConstrainedBox extends StatefulWidget {
  TestConstrainedBox({Key? key}) : super(key: key);

  @override
  State<TestConstrainedBox> createState() => _TestConstrainedBoxState();
}

class _TestConstrainedBoxState extends State<TestConstrainedBox> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Test Constrained Box")),
      body: SingleChildScrollView(
        child: Container(
          margin: const EdgeInsets.only(
              top: 20.0, left: 20.0, right: 20.0, bottom: 10.0),
          child: SizedBox.fromSize(
            size: const Size(450, 450),
            //OverflowBox(minHeight:300,minWidth:300,
            //maxWidth:300, maxHeight: 300,
            child: Center(
              child: ConstrainedBox(
                constraints: BoxConstraints(
                  minWidth: 300,
                  minHeight: 300,
                  maxWidth: 350,
                  maxHeight: 350,
                ),
                child: Card(
                  child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    crossAxisAlignment: CrossAxisAlignment.center,
                    children: [
                      //SizedBox(width:350, height:350, child:
                      Text('Hello World!')
                      //)
                    ],
                  ),
                ),
              ),
            ),
          ),
        ),
      ),
    );
  }
}