如果你以为设置preferredSize就可以，那真的是太天真了。preferredSize可以设置appbar的高度，但是无法让appbar的内容完全显示出来，因为appbar源码中kToolbarHeight常量值为56，看源码注释即可

/// The height of the toolbar component of the [AppBar].
const double kToolbarHeight = 56.0;

所以你无论怎么设置preferredSize，appbar中的的组件只显示高度范围56 两种方式来解决

1. 自定义appbar
2. 使用appbar的flexibleSpace属性

  /// This widget is stacked behind the toolbar and the tab bar. It's height will
  /// be the same as the app bar's overall height.
  ///
  /// A flexible space isn't actually flexible unless the [AppBar]'s container
  /// changes the [AppBar]'s size. A [SliverAppBar] in a [CustomScrollView]
  /// changes the [AppBar]'s height when scrolled.
  ///
  /// Typically a [FlexibleSpaceBar]. See [FlexibleSpaceBar] for details.
  final Widget flexibleSpace;