如何在DropdownButton中更改Flutter DropdownMenuItem的宽度/填充？

class CustomDropdownButtonFormField<T> extends StatefulWidget {
  final List<DropdownMenuItem<T>> items;
  final T value;
  final ValueChanged<T> onChanged;

  CustomDropdownButtonFormField({
    @required this.items,
    @required this.value,
    @required this.onChanged,
  });

  @override
  _CustomDropdownButtonFormFieldState<T> createState() =>
      _CustomDropdownButtonFormFieldState<T>();
}

class _CustomDropdownButtonFormFieldState<T>
    extends State<CustomDropdownButtonFormField<T>> {
  @override
  Widget build(BuildContext context) {
    return DropdownButtonFormField<T>(
      value: widget.value,
      items: widget.items,
      onChanged: widget.onChanged,
      decoration: InputDecoration(
        contentPadding: EdgeInsets.zero, // 取消默认的内边距
      ),
      selectedItemBuilder: (BuildContext context) {
        return widget.items.map<Widget>((DropdownMenuItem<T> item) {
          return Container(
            width: 200, // 设置宽度
            padding: EdgeInsets.symmetric(horizontal: 16, vertical: 8), // 设置填充
            child: Text(item.value.toString()),
          );
        }).toList();
      },
    );
  }
}

List<DropdownMenuItem<String>> dropdownItems = [
  DropdownMenuItem(
    value: 'Option 1',
    child: Text('Option 1'),
  ),
  DropdownMenuItem(
    value: 'Option 2',
    child: Text('Option 2'),
  ),
  DropdownMenuItem(
    value: 'Option 3',
    child: Text('Option 3'),
  ),
];

String dropdownValue = 'Option 1';

CustomDropdownButtonFormField<String>(
  items: dropdownItems,
  value: dropdownValue,
  onChanged: (String newValue) {
    setState(() {
      dropdownValue = newValue;
    });
  },
);