combobox design

What I was referring to was the old "put the component in a DisplayObject" and scale the DisplayObject up trick which seems to have been changed. Now just the ComboBox's off state is affected but the internal list is no longer affected by it.

What you want to do is actually very complicated unfortunately, simply due to nesting components inside components inside components.

A ComboBox has a List component which extends SelectableList which extends BaseScrollPane which has the _verticalScrollBar reference set to ScrollBar which is a public static CONST set to 15 for width. Yes, it's that's complicated. In order for you to actually affect the width you'll need to subclass all of these and edit the component source to create a custom version so you can simply change the static const WIDTH in ScrollBar to your desired width. You can find your components source to play with in the usual place:

Flash\Common\Configuration\Component Source\ActionScript 3.0\User Interface\fl

This is a pretty complicated thing to do, to the point you'll need to subclass so many classes, most people just go online and grab an alternate ComboBox. Just fair warning before you jump in there.

Inspect your component in the Debugger and you'll see your path to those settings (ComboBox.list._verticalScrollBar.WIDTH), but you have no access to the property, so you'll need to custom subclass to gain access.