You want to set a custom width and height for the markers on a Flex chart's legend.
The following example shows how you can set the marker width and marker height on a Flex Legend control by setting the markerWidth and markerHeight styles.
<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/09/28/setting-the-marker-width-and-height-on-a-legend-control-in-flex/ -->
<mx:Application name="Legend_markerHeight_test"
xmlns:mx="http://www.adobe.com/2006/mxml"
layout="vertical"
verticalAlign="middle"
backgroundColor="white">
<mx:XMLListCollection id="dp">
<mx:source>
<mx:XMLList>
<product label="Product 1" data="3" />
<product label="Product 2" data="1" />
<product label="Product 3" data="4" />
<product label="Product 4" data="1" />
<product label="Product 5" data="5" />
<product label="Product 6" data="9" />
</mx:XMLList>
</mx:source>
</mx:XMLListCollection>
<mx:ApplicationControlBar dock="true">
<mx:Form styleName="plain">
<mx:FormItem label="markerWidth:">
<mx:HSlider id="markerWSlider"
minimum="5"
maximum="15"
value="10"
snapInterval="1"
tickInterval="1"
liveDragging="true" />
</mx:FormItem>
<mx:FormItem label="markerHeight:">
<mx:HSlider id="markerHSlider"
minimum="5"
maximum="15"
value="10"
snapInterval="1"
tickInterval="1"
liveDragging="true" />
</mx:FormItem>
</mx:Form>
</mx:ApplicationControlBar>
<mx:Panel id="panel"
styleName="opaquePanel"
width="100%"
height="100%">
<mx:PieChart id="pieChart"
dataProvider="{dp}"
showDataTips="true"
height="100%"
width="100%">
<mx:series>
<mx:PieSeries id="pieSeries"
field="@data"
nameField="@label"
filters="[]" />
</mx:series>
</mx:PieChart>
<mx:ControlBar>
<mx:Legend id="legend"
dataProvider="{pieChart}"
direction="horizontal"
horizontalGap="100"
markerWidth="{markerWSlider.value}"
markerHeight="{markerHSlider.value}" />
</mx:ControlBar>
</mx:Panel>
</mx:Application>
For more information, see "Setting the marker width and height on a Legend control in Flex" at FlexExamples.com.