How do I auto scale down a font in a Text widget to fit the max number of lines?

An easy drop-in solution is to use FittedBox with fit: BoxFit.scaleDown:

Container(
  width: 100.0,
  child: FittedBox(
    fit: BoxFit.scaleDown,
    // Optionally apply `alignment` to position the text inside the box:
    //alignment: Alignment.topRight,
    child: SizedBox(
      child: Text('\$1,299.99'),
  )
)