Fixed width left sidebar in fluid zen theme

The solution will be different for each theme. Are you using a particular theme?

There are quite a lot of CSS changes, but to get you started, and based on this tutorial:

1. A change is required to page.tpl.php to add a wrapper div around the main content. In my example I change the page.tpl.php as follows:

<div id="main">
  <div id="content-wrapper"> <------ New div added here
.
.
    </div><!-- /#content -->
  </div><!-- /#content-wrapper --> <------ new closing div here 

2. Next are the changes to the responsive-sidebars.css style sheet. In the example below I've only shown the case where there are two sidebars and for the section '@media all and (min-width: 960px)'

/*
   * The layout when there are two sidebars.
   */
    /* Styling for the new div that was added to the page.tpl.php file */
   .two-sidebars #content-wrapper {
    float: left;
    width: 100%;
   }
   
  .two-sidebars #content { /* Span 4 columns, starting in 2nd column from left. */
    margin-left: 200px;  /* This is set to whatever width you want the left sidebar */
    margin-right: 20%;  /* This is set to whatever width you want the right sidebar */
  }

  .two-sidebars .region-sidebar-first { /* Span 1 column, starting in 1st column from left. */
    float: left;
    width: 200px;  /* This is set to whatever width you want the left sidebar */
    margin-left: -100%;
  }
  .two-sidebars .region-sidebar-second { /* Span 1 column, starting in 5th column from left. */
    float: left;
    width: 20%; /* This is set to whatever width you want the right sidebar */
    margin-left: -20%; /* This is set to whatever width you want the right sidebar */
  }

Obviously you'll need to make appropriate changes to the style sheet for the other screen sizes.

Hope this helps

Ed