on the open event of the combobox you can set dropDown.x =dropDown.x-(dropDown.width-combobox.width) and it will move the dropDown left by the difference amount
This is a good candidate for an enhancement request, I've logged one here http://bugs.adobe.com/jira/browse/SDK-28698
One possible work-around is to let the width of the dropDown be the same as the PopUpAnchor, but wrap the content in a second Group and right-align it to the dropDown. Then the skin would be something like this:
<!-- This will be as wide as the comboBox -->
<!-- Add a second group and align it to the 'right' inside of the first group -->
<s:Group right="0" top="0" bottom="0">
The disadvantage of this method is that if the combo-box is too close to the left edge of the screen and the dropDown content is wider than the comboBox, it will be clipped (the PopUpAnchor won't detect this case and auto-shift the position).