使用FractionallySizedBox和Alignment bottomCenter。
示例代码：

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData.dark(
        useMaterial3: true,
      ),
      home: const MyHomePage(),
      debugShowCheckedModeBanner: false,
    );
  }
}

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

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int height = 200;

  void _onRemovePressed() {
    if (height <= 100) {
      return;
    }
    setState(() {
      height -= 50;
    });
  }

  void _onAddPressed() {
    if (height > MediaQuery.sizeOf(context).height - 100) {
      return;
    }
    setState(() {
      height += 50;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: const Text('Demo'),
        actions: [
          IconButton(
            onPressed: _onAddPressed,
            icon: const Icon(Icons.add),
          ),
          IconButton(
            onPressed: _onRemovePressed,
            icon: const Icon(Icons.remove),
          )
        ],
      ),
      //actual code starts here
      body: SizedBox(
        height: height.toDouble(),
        width: double.infinity,
        child: FractionallySizedBox(
          alignment: Alignment.bottomCenter,
          heightFactor: 0.5,
          child: ColoredBox(
            color: Colors.purple.shade200,
          ),
        ),
      ),
    );
  }
}