<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/07/02/setting-the-width-of-the-dropdown-menu-on-a-combobox-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="top"
        backgroundColor="white">
 
    <mx:Array id="arr">
        <mx:Object label="One" />
        <mx:Object label="Two" />
        <mx:Object label="Three" />
        <mx:Object label="Four" />
        <mx:Object label="Five" />
        <mx:Object label="Six" />
        <mx:Object label="Seven" />
        <mx:Object label="Eight" />
        <mx:Object label="Nine" />
        <mx:Object label="Ten" />
    </mx:Array>
 
    <mx:ComboBox id="comboBox"
            dataProvider="{arr}"
            dropdownWidth="200" />
 
</mx:Application>

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/07/02/setting-the-width-of-the-dropdown-menu-on-a-combobox-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="top"
        backgroundColor="white"
        initialize="init();">
 
    <mx:Script>
        <![CDATA[
            import mx.controls.ComboBox;
 
            private var arr:Array;
            private var comboBox:ComboBox;
 
            private function init():void {
                arr = [];
                arr.push({label:"One"});
                arr.push({label:"Two"});
                arr.push({label:"Three"});
                arr.push({label:"Four"});
                arr.push({label:"Five"});
                arr.push({label:"Six"});
                arr.push({label:"Seven"});
                arr.push({label:"Eight"});
                arr.push({label:"Nine"});
                arr.push({label:"Ten"});
 
                comboBox = new ComboBox();
                comboBox.dataProvider = arr;
                comboBox.dropdownWidth = 200;
                addChild(comboBox);
            }
        ]]>
    </mx:Script>
 
</mx:Application>