可以使用PreferredSize来设置AppBar的高度，并在其中包含一个FlexibleSpaceBar来调整CircleAvatar的大小。例如：

AppBar( automaticallyImplyLeading: false, backgroundColor: Colors.white, brightness: Brightness.light, elevation: 0, centerTitle: true, title: Text( "My App", style: TextStyle( color: Colors.black, fontSize: 20 ), ), bottom: PreferredSize( child: FlexibleSpaceBar( background: Container( color: Colors.white, child: Column( mainAxisAlignment: MainAxisAlignment.center, children: [ CircleAvatar( backgroundImage: AssetImage('assets/images/avatar.png'), radius: 30, ), SizedBox(height: 10), Text( "Username", style: TextStyle( color: Colors.black, fontSize: 16, ), ), ], ), ), ), preferredSize: Size.fromHeight(100), ), );