I had a Flex Combobox that had contents too big to fit, but when I made it shorter to fit, the items would be cut off. There’s a brilliant solution on the FlexExamples site that handles it so:
<mx:ComboBox id="projectComboBox" dataProvider="{projectList}" maxWidth="400" itemRenderer="mx.controls.Label"/>
All this does is specify the use of a label control as the TextField in the ComboBox. Since Labels are made to truncate then produce tooltips automatically, this is amazingly simple.