Scaling issue in fluid layout

I am sure this is an easy one but I can’t figure it out:

I have a movieclip (tester) that I am locating on the stage via the following code:

setStage();

var stageListener:Object = new Object();

Stage.addListener(stageListener);
stageListener.onResize = function() {
setStage();
};

function setStage() {
var WIDTH:Number = Stage.width;
var HEIGHT:Number = Stage.height;

tester._x = Stage.width/2;
tester._y = (Stage.height + -70)/2;

What I want to have happen is have it start in that location but when you drag the browser window open the “tester” movieclip scales in place to a certain percentage - basically as the window gets bigger I want it to stay in the same place but get bigger and be able to control how big as the window opens.

Can anyone help out…

Thank you