How can I center a Positioned while also making it wide?

So I have a Positioned widget (the DottedBorder widget provided by the package of the same name is a Positioned):

Column(
  crossAxisAlignment: CrossAxisAlignment.center,
  children: <Widget>[
    DottedBorder(
      borderType: BorderType.RRect,
      color: Colors.black,//color of dotted/dash line
      strokeWidth: 3, //thickness of dash/dots
      dashPattern: [10,6], 
      //dash patterns, 10 is dash width, 6 is space width
      radius: const Radius.elliptical(10.0, 10.0),
      child: Padding(
        padding: EdgeInsets.fromLTRB(3.0, 3.0, 1060.0, 3.0),           
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.end,
          textDirection: TextDirection.ltr,
          children: <Widget>[
            Text("Welcome!",
              textAlign: TextAlign.left,
              style: TextStyle(
                fontWeight: FontWeight.bold,
                fontSize: 20.0),),
          ],
        ),
      ),
    )
  ],
),

As you can see, it is extremely padded to the right to make it looks somewhat centered.

However, I want it to be accurately centered, while also spreading across the page and keeping the text inside on the left. How can I do that? Thanks.

Use crossAxisAlignment: CrossAxisAlignment.stretch and remove large left padding

 Column(
  mainAxisAlignment: MainAxisAlignment.center,
  crossAxisAlignment: CrossAxisAlignment.stretch,
  mainAxisSize: MainAxisSize.max,
  children: <Widget>[
    DottedBorder(
      borderType: BorderType.RRect,
      color: Colors.black, //color of dotted/dash line
      strokeWidth: 3, //thickness of dash/dots
      dashPattern: [10, 6],
      radius: const Radius.elliptical(10.0, 10.0),
      child: Padding(
        padding: EdgeInsets.fromLTRB(3.0, 3.0, 3.0, 3.0), //>> padding
        child: Column(
          textDirection: TextDirection.ltr,
          children: <Widget>[
            Text(
              "Welcome!",
              textAlign: TextAlign.left,
              style:
                  TextStyle(fontWeight: FontWeight.bold, fontSize: 20.0),
            ),
          ],
        ),
      ),
    )
  ],
),