I don't know how to do it in Starling Builder, but in AS3, if you set both
right on a child's
AnchorLayoutData object, the child will resize to fill the entire width of the container.
var anchorLayoutData:AnchorLayoutData = new AnchorLayoutData()
anchorLayoutData.left = 0;
anchorLayoutData.right = 0;
child.layoutData = anchorLayoutData;
This is exactly the sort of thing that
AnchorLayout was designed for.