Flutter: How to make a button expand to the size of its parent?

Wrapping with a ButtonTheme with minWidth: double.infinity allows to provide constraints

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'),
    ),
  ),

Add the crossAxisAlignment property to your Row;

crossAxisAlignment: CrossAxisAlignment.stretch

We can add Button insider Container.

Solution:

Container(
            width: double.infinity,
            child: RaisedButton(
              onPressed: null,
              child: Text('NEXT'),
            ),
          )