SizedBox控件能强制子控件具有特定宽度、高度或两者都有

import 'package:flutter/material.dart';
class LayoutDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('强制大小'),),body: new SizedBox(
        width: 250.0,height: 250.0,child: new Container(
          decoration: new BoxDecoration(
            backgroundColor: Colors.lightBlueAccent[100],);
  }
}
void main() {
  runApp(
    new MaterialApp(
      title: 'Flutter教程',home: new LayoutDemo(),);
}

AspectRatio控件能强制子小部件的宽度和高度具有给定的宽高比，以宽度与高度的比例表示。

import 'package:flutter/material.dart';
class LayoutDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('强制比例'),body: new AspectRatio(
        aspectRatio: 3.0 / 2.0,);
}