Create an HBox that is 400 pixels wide, with a Button inside set to a width of 50 percent
<?xml version="1.0" encoding="utf-8"?> <mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute"> <mx:HBox width="400"> <mx:Button label="Button" width="50%" /> </mx:HBox> </mx:Application>