Expanded(
      child: Image.asset('images/pic3.jpg'),
    ),
  ],
);

App source: sizing

Perhaps you want a widget to occupy twice as much space as its
siblings. For this, use the Expanded widget flex property,
an integer that determines the flex factor for a widget.
The default flex factor is 1. The following code sets
the flex factor of the middle image to 2:

flex: 2,
      child: Image.asset('images/pic2.jpg'),
    ),
    Expanded(
      child: Image.asset('images/pic3.jpg'),
    ),
  ],
);

App source: sizing

Packing widgets