dropDownWidth is what you want.
Depending on how often you do this, the Flextras AutoCompleteComboBox implements an expand drop down to content property which automatically calculates the width of the longest item in your ComboBox and expand the drop down to that width. Along with that we also implement a truncate to fit property.
More info here:http://www.flextras.com/?event=ProductHome&productID=10
You can enable / disable the properties on this sample to see how they affect the component: http://www.flextras.com/AutoCompleteComboBox/Samples/CodeGenerator/ and/or register to download our no cost developer edition and test it out yourself.