Need different sidebar lengths

Creating a website in CS4 with many pages. Some pages have a lot of content and some have a little content.

When I change the length of the sidebar on the page with a lot of content using CSS it changes the length of the pages with a little content.

I need to know how to independently change the length of the sidebar for the pages that have a lot of content, while not changing the length of the pages with little content.

Most pages have a small amount of content.

Thank you in advance.