you need to wrap your Column with an IntrinsicWidth

example (try to run this):

return IntrinsicWidth(
  child: Column(
    mainAxisAlignment: MainAxisAlignment.center,
    crossAxisAlignment: CrossAxisAlignment.start,
    children: [
      Row(
        children: [
          const Spacer(),
          Container(
            color: Colors.white,
            width: 20,
            height: 40,
          ),
        ],
      ),
      Container(
        color: Colors.orange,
        width: 50,
        height: 40,
      ),
      Container(
        color: Colors.white,
        width: 100,
        height: 20,
      ),
    ],
  ),
);