Make sure you are using the latest version of Opus (10.0.2.0) and that the Auto-size columns option is turned on for the folder format.
(Via the Folder Options window, Display tab; or via the Folder Formats section in Preferences. See the Folder Format FAQs near the top of the list for more info if you have problems applying the change to all folders.)
Thanks again leo. As it turned out, the Auto-size columns option had been checked (this must be the default setting, since I hadn't touched it). However, upgrading from 10.0.1.0 to 10.0.2.0 seems to have cured the problem.