[kup]

А что у тебя за хелп такой?! Я перед тем как спросить, конечно прочитал, что об этом думает хелп:

stageWidth property

stageWidth:int []

Language Version : ActionScript 3.0
Runtime Versions : AIR 1.0, Flash Player 9

Specifies the current width, in pixels, of the Stage.

If the value of the Stage.scaleMode property is set to StageScaleMode.NO_SCALE when the user resizes the window, the Stage content maintains its defined size while the stageWidth property changes to reflect the new width size of the screen area occupied by the SWF file. (In the other scale modes, the stageWidth property always reflects the original width of the SWF file.) You can add an event listener for the resize event and then use the stageWidth property of the Stage class to determine the actual pixel dimension of the resized Flash Player window. The event listener allows you to control how the screen content adjusts when the user resizes the window.

Note: In an HTML page hosting the SWF file, both the object and embed tags' width attributes must be set to a percentage (such as 100%), not pixels. If the settings are generated by JavaScript code, the width parameter of the AC_FL_RunContent() method must be set to a percentage, too. This percentage is applied to the stageWidth value.

Implementation
public function get stageWidth():int
public function set stageWidth(value:int):void

Throws
SecurityError — Calling the stageWidth property of a Stage object throws an exception for any caller that is not in the same security sandbox as the Stage owner (the main SWF file). To avoid this, the Stage owner can grant permission to the domain of the caller by calling the Security.allowDomain() method or the Security.allowInsecureDomain() method. For more information, see the "Security" chapter in Programming ActionScript 3.0.

See also
flash.display.StageScaleMode