num width

The width of this display object with the applied transformation.

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

Source 

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

Source 

set width(num value) {
  this.scaleX = 1.0;
  num normalWidth = this.width;
  this.scaleX = (normalWidth != 0.0) ? value / normalWidth : 1.0;
}