If you make a combo box 25 pixels wide because you are trying to save screen real estate, the popup will be only 25 pixels wide.
This is addressed now in RFE with Id 4618607.
This is a bug and it has been annoying me for some time. It needs to be fixed.
Will not fix. I don't see this as a bug. The popup portion should not exceed the width of the visible portion of the combo box. The submitter and evaluator (one and the same) doesn't offer a constructive suggestion.