A simple demo page of updating box size.

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(const MaterialApp(home: AnimationPage()));
}

class AnimationPage extends StatefulWidget {
  const AnimationPage({super.key});

  @override
  State<AnimationPage> createState() => _AnimationPageState();
}

class _AnimationPageState extends State<AnimationPage> {
  double _size = 50;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        children: [
          CustomPaint(size: Size.infinite, painter: _GridPainter()),
          Positioned(
            left: 20,
            top: 20,
            child: Container(width: _size, height: _size, color: Colors.blue),
          ),
        ],
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          setState(() => _size = 100);
        },
      ),
    );
  }
}

class _GridPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final paint =
        Paint()
          ..color = Colors.grey.withOpacity(0.3)
          ..strokeWidth = 0.5;

    // Draw vertical lines
    for (var i = 0.0; i < size.width; i += 10) {
      canvas.drawLine(Offset(i, 0), Offset(i, size.height), paint);
    }

    // Draw horizontal lines
    for (var i = 0.0; i < size.height; i += 10) {
      canvas.drawLine(Offset(0, i), Offset(size.width, i), paint);
    }
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) => false;
}