double widthFactor

If non-null, the factor of the incoming width to use.

If non-null, the child is given a tight width constraint that is the max incoming width constraint multipled by this factor. If null, the child is given the incoming width constraints.

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();
}