How to clear the explicit width and height of a Group - apache-flex

Question

I have a group that sizes to the contents it contains but to perform a transition using Tweener I have to set the width and height properties. After the animation is done how would I clear the explicit width and height properties so that the Group goes back to sizing based on it's contents?
Before:
<s:Group ><content.../></s:Group>
Code:
width = 250;
height = 250;
Tweener.addTween(this, {width:500, height:500, time:0.25, onComplete:openAnimationComplete});
After the tween the Group is effectively set to 500x500 as if it were this:
<s:Group width="500" height="500" ><content /></s:Group>
I want it to return to this:
<s:Group ><content /></s:Group>
UPDATE
Test code using solution from Flextras:
<fx:Script>
<![CDATA[
protected function button1_clickHandler(event:MouseEvent):void
{
var w:Object = myGroup.width;
var h:Object = myGroup.height;
if (myGroup.width!=300) {
myGroup.width = 300;
myGroup.height = 300;
}
else {
myGroup.width = NaN;
myGroup.height = NaN;
w = myGroup.width; // NaN
h = myGroup.height; // NaN
//myGroup.validateNow()
if (myGroup.parent is IInvalidating) {
IInvalidating(myGroup.parent).validateNow();
}
w = myGroup.width; // 600
h = myGroup.height; // 100
}
}
]]>
</fx:Script>
<fx:Declarations>
<!-- Place non-visual elements (e.g., services, value objects) here -->
</fx:Declarations>
<s:Group id="myGroup" minHeight="0" minWidth="0" clipAndEnableScrolling="true">
<s:Button width="600" height="100" label="here is a button" click="button1_clickHandler(event)"/>
</s:Group>

Answer 1

Set the height and width to NaN; which would effectively tell the Flex Framework "I have no idea what this value is" and will then execute the measure() event during the next time the component renders itself; causing it to update the measuredHeight and measuredWidth properties; which will in turn be used by the parent container to size the group.