double textScaleFactor

The number of font pixels for each logical pixel.

For example, if the text scale factor is 1.5, text will be 50% larger than the specified font size.

Source 

double get textScaleFactor => _textPainter.textScaleFactor;
void textScaleFactor=(double value)

Source 

set textScaleFactor(double value) {
  assert(value != null);
  if (_textPainter.textScaleFactor == value)
    return;
  _textPainter.textScaleFactor = value;
  _overflowShader = null;
  markNeedsLayout();
}