这里有一个方法来解决您的问题

Widget customDropDown(BuildContext context) {
    return Container(
      /// Set the width of the DropdownButton
      width: MediaQuery.of(context).size.width,
      margin: const EdgeInsets.all(15),
      child: PopupMenuButton<String>(
        child: Container(
          padding: EdgeInsets.all(10),
          decoration: BoxDecoration(
            /// here you can control border and it's radius
            border: Border.all(color: Colors.grey),
            borderRadius: BorderRadius.circular(4),
          ),
          child: Row(
            children: [
              Expanded(
                child: Text(
                  _selectedItem.isNotEmpty ? _selectedItem : 'Select an item',
                  overflow: TextOverflow.ellipsis,
                ),
              ),
              Icon(Icons.arrow_drop_down),
            ],
          ),
        ),
        offset: Offset(0, 40), // Adjust the offset as needed
        itemBuilder: (BuildContext context) => <PopupMenuEntry<String>>[
          PopupMenuItem<String>(
            value: 'Option 1',
            child: Container(
              width: 120, // Set the width of the PopupMenuItem
              child: Text('Option 1'),
            ),
          ),
          PopupMenuItem<String>(
            value: 'Option 2',
            child: Container(
              width: 120, // Set the width of the PopupMenuItem
              child: Text('Option 2'),
            ),
          ),
          PopupMenuItem<String>(
            value: 'Option 3',
            child: Container(
              width: 120, // Set the width of the PopupMenuItem
              child: Text('Option 3'),
            ),
          ),
        ],
        onSelected: (String value) {
          /// TODO Your Method
        },
      ),
    );
  }