num height

The height of this display object with the applied transformation.

The height is calculated based on the bounds of the content of the display object. When you set the width property, the scaleY property is adjusted accordingly.

Source 

num get height => this.boundsTransformed.height;
void height=(num value)

Source 

set height(num value) {
  this.scaleY = 1.0;
  num normalHeight = this.height;
  this.scaleY = (normalHeight != 0.0) ? value / normalHeight : 1.0;
}