How to make button width match parent?

Update:

With Flutter 2.0 RaisedButton is deprecated and replaced by ElevatedButton. you can use minimumSize like this:

ElevatedButton(
        style: ElevatedButton.styleFrom(
          minimumSize: Size.fromHeight(40), // fromHeight use double.infinity as width and 40 is the height
        ),
        onPressed: () {},
        child: Text('Text Of Button'),
      )

Old answer for Flutter less than 2.0:

The correct solution would be to use the SizedBox.expand widget, which enforces its child to match its parent’s size.

SizedBox.expand(
  child: RaisedButton(...),
)

There are many alternatives, which allows for more or less customization:

SizedBox(
  width: double.infinity,
  // height: double.infinity,
  child: RaisedButton(...),
)

or using a ConstrainedBox

ConstrainedBox(
    constraints: const BoxConstraints(minWidth: double.infinity),
    child: RaisedButton(...),
)