Flutter : Getting width and height after Tansform using Matrix4

I'm still learning flutter and I was working on an app in which I need to do some perspective manipulation on images, I played with the Transform widget using Matrix4 and I was able to rotate a container along Y axis, here is the result :

Before animation

After Animation

As you can see the container rotated along Y axis in the 3D Space but practically at the level of the 2D plane of the screen its height shrinked, what I want here is to get the height of the container after it shrinked. I tried to get its height using RenderBox but it only gave me the height before Animation.

Here is my code :

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

void main() {
  runApp(MyApp());
}
GlobalKey containerKey = GlobalKey();

class MyApp extends StatelessWidget {
  MyApp({Key? key}) : super(key: key);
  @override
  Widget build(BuildContext context) {
    return ScreenUtilInit(
      designSize: const Size(1440, 3040),
      minTextAdapt: true,
      splitScreenMode: true,
      builder: (context, child) {
        return MaterialApp(
          debugShowCheckedModeBanner: false,
          theme: ThemeData(
            primarySwatch: Colors.blue,
          ),
          home: child,
        );
      },
      child: const MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key}) : super(key: key);
  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return const Scaffold(
      body: Center(
        child: Rect(),
      ),
    );
  }
}

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

  @override
  State<Rect> createState() => _RectState();
}

class _RectState extends State<Rect> with SingleTickerProviderStateMixin {
  late AnimationController _controller;
  bool isDone = false;
  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
      duration: Duration(seconds: 2),
        vsync: this
    );

  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  getSize(GlobalKey key){
    WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
      RenderBox renderBox = key.currentContext?.findRenderObject() as RenderBox;
      final Size size = renderBox.size;
      print('Size: ${size.width}, ${size.height}');
    });
  }

  @override
  Widget build(BuildContext context) {

    return AnimatedBuilder(
      animation: _controller,
      builder: (BuildContext context, Widget? child) {
        _controller.addStatusListener((status) {
          if(status == AnimationStatus.completed){
            getSize(containerKey);
          }
        });
        return Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          crossAxisAlignment: CrossAxisAlignment.center,
          children: [
            Transform(
              transform: Matrix4.identity()..rotateX(_controller.value * 2*pi/8),
              alignment: FractionalOffset.center,
              child: Container(
                key: containerKey,
                width: 400.w,
                height: 400.h,
                color: Colors.redAccent,
              ),
            ),
            ElevatedButton(
              onPressed: () {
                if (!isDone) {
                  _controller.forward();
                  isDone = !isDone;
                } else {
                  _controller.reverse();
                  isDone = !isDone;
                }
              },
              child: Text('Animate !'),
              style: ElevatedButton.styleFrom(
                  shadowColor: Colors.black,
                  shape: StadiumBorder(),
                  padding: EdgeInsets.symmetric(horizontal: 35, vertical: 20)),
            )
          ],
        );
      },
    );
  }
}