Button Width Match Parent

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(...),
)

The size attribute can be provided using ButtonTheme with minWidth: double.infinity

ButtonTheme(
  minWidth: double.infinity,
  child: MaterialButton(
    onPressed: () {},
    child: Text('Raised Button'),
  ),
),

or after https://github.com/flutter/flutter/pull/19416 landed

MaterialButton(
  onPressed: () {},
  child: SizedBox.expand(
    width: double.infinity, 
    child: Text('Raised Button'),
  ),
),

Container(
  width: double.infinity,
  child: RaisedButton(...),
),

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