The following tutorial covers changing diazo Flow Plone Theme columns width on a Plone 4.2 website.
Flow diazo theme has customizable columns width. Default columns width is: 221px - for left column, 175px for right column.
You can globally change default columns width via Diazo Theme : Site Setup -> Diazo Theme -> Advanced Settings.
Note: For Diazo Support extension starting with 1.1b1 version, proceed to Site Setup ->Theming -> Advanced Settings tab.
In Parameter expressions textarea fill in the desirable values into columnonewidth and columntwowidth fields. IMPORTANT: To change columns width - you should specify new values in both fields (even if you want to update only one column width, as in the example below).
Save changes and return to Plone interface to view them: