you can't do what you are trying to do the way you are trying to do it.

I have included some code here (pure AS3, no timeline or library) which you might be able to adapt to your needs

Basically I have created a static property (_rectWidth which controls the rectangles width) on the MovieClip class that contains the rectangle and a render() function that redraws the rectangle tot he correct width. Note you will have to call render() after setting a new width to see the rectangle drawn at the new  width.

Then in the main class I create 5 rectangles with random colours and positions.

The main class also listens for the "arrow up" and "arrow down" keys to change the rectangle's size. Note: I change the size of the rectangle and then run through all of the Main classes children (which are all RectMC's) and call render();

package 
{
          import flash.display.Sprite;
          import flash.events.Event;
          import flash.events.KeyboardEvent;
 
          public class Main extends Sprite 
          {
 
                    public function Main():void 
                    {
                              if (stage) init();
                              else addEventListener(Event.ADDED_TO_STAGE, init);
                    }
 
                    private function init(e:Event = null):void 
                    {
                              removeEventListener(Event.ADDED_TO_STAGE, init);
                              // entry point
 
                              for (var i:int = 0; i < 5; i++)
                              {
                                        var col:uint = Math.floor(Math.random() * 0xffffff);
                                        var rectX:int = Math.floor(Math.random() * stage.stageWidth);
                                        var rectY:int = Math.floor(Math.random() * stage.stageHeight);
                                        var rectMC:RectMC = new RectMC(col);
                                        rectMC.x = rectX;
                                        rectMC.y = rectY;
                                        addChild(rectMC);
                              }
 
                              stage.addEventListener(KeyboardEvent.KEY_DOWN, onKeyDown);
                    }
 
                    private function onKeyDown(e:KeyboardEvent):void 
                    {
                              switch (e.keyCode)
                              {
                                        case 38:
                                                  RectMC.rectWidth += 20;
                                                  break;
 
                                        case 40:
                                                  RectMC.rectWidth -= 20;
                                                  break;
                              }
 
                              for (var i:int = 0; i < numChildren; i++)
                              {
                                        var rectMC:RectMC = getChildAt(i) as RectMC;
                                        rectMC.render();
                              }
                    }
 
          }
 
}
 

package  
{
          import flash.display.MovieClip;
          import flash.events.Event;
 
          /**
           * ...
           * @author steven O'Boyle
           */
          public class RectMC extends MovieClip 
          {
                    protected static var _rectWidth:int = 100;
                    protected static var _rectHeight:int = 50;
 
                    protected var _rectColour:uint;
 
                    public function RectMC(col:uint) 
                    {
                              _rectColour = col;
                              render();
                    }
 
                    public function render():void
                    {
                              graphics.clear();
 
                              graphics.beginFill(_rectColour);
                              graphics.drawRect(0, 0, _rectWidth, _rectHeight);
                              graphics.endFill();
                    }
 
                    public static function set rectWidth(value:int):void
                    {
                              _rectWidth = value;
                    }
 
                    public static function get rectWidth():int
                    {
                              return _rectWidth;
                    }
          }
 
 
}