Making the content window 'relative' in Barlow theme -- please help

Hello -- I having a problem with my Drupal site: it's working great, with one small annoyance... If I resize the browser window to a small size, (which I often do), the content window goes underneath the left sidebar. I would rather have it shrink the content window while keeping the position intact.

Is there a simple way to change this in style.css or any other of the theme files? .

Link: http://www.youronlineprofessor.net/

Please advise... Thanks!