Card width match with parent : flutter

Just Put Card in Container

Padding(
            padding: EdgeInsets.all(20),
            child: Container(
              height: 50,
              width: double.infinity,
              child: Card(
                child: Align(
                    alignment: Alignment.centerLeft,
                    child: Text("Edit Profile")),
              ),
            ),
          )

If you want to resize or alter the card view Then just put Card inside the Container view then adjust your container size. This link will help you :https://stackoverflow.com/a/50017126/7418129

You don't need Stack to get that view - it can be done using Column & Material widget.

return Scaffold(
        body: new Container(
      decoration: new BoxDecoration(color: Colors.blue),
      child: new Center(
          child: MergeSemantics(
            child: Padding(
              padding: const EdgeInsets.all(16.0),
              child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: <Widget>[
              Card(
                child: Column(
                  crossAxisAlignment: CrossAxisAlignment.stretch,
                  children: <Widget>[
                    Material(
                      elevation: 24.0,
                      child: Padding(
                        padding: const EdgeInsets.all(20.0),
                        child: Text("Sign Up Here"),
                      ),
                    ),
                    Padding(
                        padding: EdgeInsets.fromLTRB(20, 10.0, 20.0, 20.0),
                        child: Column(
                          crossAxisAlignment: CrossAxisAlignment.stretch,
                          children: <Widget>[
                            TextField(
                              decoration: InputDecoration(
                                labelText: "Email",
                                hintText: "[email protected]",
                              ),
                              autofocus: true,
                            ),
                            TextField(
                              decoration: InputDecoration(
                                labelText: "Password",
                              ),
                              autofocus: true,
                            ),
                            SizedBox(
                              width: double.infinity,
                              // height: double.infinity,
                              child: new RaisedButton(
                                color: Colors.blue,
                                child: Text(
                                  "Sign In",
                                  style: TextStyle(
                                    color: Colors.white,
                                    fontFamily: 'Raleway',
                                    fontSize: 22.0,
                                  ),
                                ),
                                onPressed: () => print('Sign In'),
                              ),
                            )
                          ],
                        )),
                  ],
                ),
              ),
        ],
      ),
            ),
          )),
    ));

Output: