FractionallySizedBox({Key key, FractionalOffset alignment: FractionalOffset.center, double widthFactor, double heightFactor, Widget child })

Creates a widget that sizes its child to a fraction of the total available space.

If non-null, the widthFactor and heightFactor arguments must be non-negative.

Source 

FractionallySizedBox({
  Key key,
  this.alignment: FractionalOffset.center,
  this.widthFactor,
  this.heightFactor,
  Widget child
}) : super(key: key, child: child) {
  assert(alignment != null && alignment.dx != null && alignment.dy != null);
  assert(widthFactor == null || widthFactor >= 0.0);
  assert(heightFactor == null || heightFactor >= 0.0);
}