Forum display width
How can I remove the white space on the right side of the forum window. I would like the forum window to occupy 100% of the screen size.
Please see attachment for details.
Put some widget in main sidebar (try with all sidebars) and remove it. This will reset sidebar cache and it'll be removed. However on some themes it's hard-coded and may not be removed. You should look for Full Width page template in Edit page screen and change it.
In any case non of above described have direct relation to wpForo. All are theme/page template/sidebar issues.