Сделал, так :

Код AS3:
<?xml version="1.0" encoding="utf-8"?>
<s:Label xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:mx="library://ns.adobe.com/flex/mx"
addedToStage="onAddedToStage()"
backgroundColor="#FF0000">
<fx:Declarations>
<!-- Place non-visual elements (e.g., services, value objects) here -->
</fx:Declarations>
<fx:Script >
<![CDATA[
private var _width:Number;
private var _height:Number;
protected function onAddedToStage():void
{
width = _width;
height = _height;
}
override public function set width(value:Number):void
{
trace("set width : " + value);
if (isNaN(value)) return;
_width = value;
var boundsWidth:Number = getBounds(this).width;
if (!boundsWidth)
{
return;
}
scaleX = _width / boundsWidth;
}
override public function set height(value:Number):void
{
trace("set height : " + value);
if (isNaN(value)) return;
_height = value;
var boundsHeight:Number = getBounds(this).height;
if (!boundsHeight)
{
return;
}
scaleY = _height / boundsHeight;
}
]]>
</fx:Script>
</s:Label>