double widthFactor

If non-null, sets its width to the child's width multipled by this factor.

Can be both greater and less than 1.0 but must be positive.

Source 

double get widthFactor => _widthFactor;
void widthFactor=(double value)

Source 

set widthFactor (double value) {
  assert(value == null || value >= 0.0);
  if (_widthFactor == value)
    return;
  _widthFactor = value;
  markNeedsLayout();
}