CODE EXAMPLE FOR DART

flutter add height to appbar

appBar: PreferredSize(
  preferredSize: Size.fromHeight(200.0),
  child: AppBar(
    automaticallyImplyLeading: false,
    flexibleSpace:  Image(
              image: AssetImage('assets/images/banner_image.jpg'),
              fit: BoxFit.cover,
            ),
            backgroundColor: Colors.transparent,
  )
),