Overflow for fixed layout setting (center aligned 960x)

I have content on the left sidebar and right sidebar and on some of my screens the main content is overflowing into the right sidebar. I have set the overflow property to auto so that I can see the content but what I want to do is change the size of the right and left sidebars so that my content can always be seen. This is happening on my permissions screen which is a big pain in the butt and its happening on my webform screens as well.

I am really new to website design and Drupal so any information will be of help. Thanks.