How do I autosize Movieclip to the height of textfield

this.createEmptyMovieClip("toolTip", 10);
function drawRectangle(target_mc:MovieClip, boxWidth:Number, boxHeight:Number, fillColor:Number, fillAlpha:Number):Void {
    with (target_mc) {
        lineStyle(2,0x000000,100);
        beginFill(fillColor, fillAlpha);
        moveTo(0, 0);
        lineTo(boxWidth, 0);
        lineTo(boxWidth, boxHeight);
        lineTo(0, boxHeight);
        lineTo(0, 0);
        endFill();
}
var tipText:TextField = toolTip.createTextField("tipText", this.getNextHighestDepth(), 4, 4, boxWidth-4, boxHeight-4);
        tipText.multiline = true;
        tipText.wordWrap = true;
        tipText.autoSize = true;
        tipText.text = "New articles available on Developer Center. The preceding code dynamically creates a new text field, enables HTML formatting, and displays the text 'New articles available on Developer Center' on the Stage, with the word 'Developer Center' appearing in italics.";
 
var autoHeight = tipText.textHeight;
		}
drawRectangle(toolTip, 240, autoHeight, 0x99FF00, 100);