将其 Package 在Container中，因此子节点也获得父节点的大小。

Container(
      width: 400,
      child: ElevatedButton(
        
        style: ButtonStyle(
            shape: MaterialStateProperty.all<RoundedRectangleBorder>(
                RoundedRectangleBorder(
                    borderRadius: BorderRadius.circular(18.0),
                    side: BorderSide()))),
        onPressed: () {},
        child: const Text(
          "Baca",
          style: TextStyle(color: Colors.white),
        ),
      ),
    );

请尝试以下ElevatedButton宽度和高度的答案：

ElevatedButton(
    onPressed: () {},
    child: Text("Ok"),
    style: ElevatedButton.styleFrom(
        //change width and height on your need width = 200 and height = 50
    minimumSize: Size(200, 50),
    ),
),

最好像这样使用SizedBox：

SizedBox(
  width: 200,
  child: ElevatedButton(
    style: ButtonStyle(
      shape: MaterialStateProperty.all<RoundedRectangleBorder>(
        RoundedRectangleBorder(
          borderRadius: BorderRadius.circular(18.0),
          side: BorderSide()
        )
      )
    ),
    onPressed: () {},
    child: const Text(
      "Baca",
      style: TextStyle(color: Colors.white),
    ),
  ),
);