<mx:Panel
    xmlns:fx="http://ns.adobe.com/mxml/2009"
    xmlns:mx="library://ns.adobe.com/flex/mx"
    xmlns:s="library://ns.adobe.com/flex/spark" >

	<mx:ComboBox id="theCB" width="150" selectedIndex="0" open="cb_openHandler(event)" >
    <fx:Script>
        <![CDATA[
            import mx.events.DropdownEvent;

            protected function cb_openHandler(event:DropdownEvent):void
            {
                theCB.dropdown.height = 150;
            }

        ]]>
    </fx:Script>
		<mx:dataProvider>
			<mx:ArrayCollection>
				<fx:String>Product A</fx:String>
				<fx:String>Product B</fx:String>
				<fx:String>Product C</fx:String>
				<fx:String>Product D</fx:String>
				<fx:String>Product E</fx:String>

			</mx:ArrayCollection>
		</mx:dataProvider>

	</mx:ComboBox>

</mx:Panel>