the forum uses the width given to page content by your wp theme…  these days, most themes come with a full width template that will remove sidebars and give more width to the forum (however if based on wp twentyten or twentyeleven their full widths just add whitespace on sides)…  if your theme does not, you will need to create your own template to allocate more width…  the wp codex: http://codex.wordpress.org/Pages can help you create your own custom page template…

if you think its something else, we would need a link to your site to understand better…

Thanks very much for your reply. It sits on a password-protected page within the Optimize Press theme, I didn’t know if there were a couple of values I could change easily. I think I might just let it be rather than risk breaking my site!

changing the forum page to work on a full-width template should have no disadvantageous effect if ones comes with the WP theme.