In AS3 stage.width works BUT it gives you the measurement of all the items in the stage - not the width of the player window.
This person has an interesting workaround. Basically you create an empty movieclip, convert it to a Stage, then measure it. Because there is nothing in it, it measures what it can, so measures the player window!
At least that is how I understand it. Here is the code:
var myMC:MovieClip = new MovieClip();
var _stage:Stage = myMC.stage;
Then you use this to get the measurement:
This even works in SWFs that are loaded in a parent shell SWF.