Card(
  shape: RoundedRectangleBorder(
    borderRadius: BorderRadius.circular(40), // if you need this
    side: BorderSide(
      color: Colors.grey.withOpacity(0.2),
      width: 1,
    ),
  ),
  child: Container(
    color: Colors.white,
    width: 200,
    height: 200,
  ),
)