double heightFactor

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

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

Source 

double get heightFactor => _heightFactor;
void heightFactor=(double value)

Source 

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