It can’t really be done that way without some javascript enhancements. Its not how CSS works. A common way to make a sidebar look the same height as the content area is to fake it using a technique called faux columns – applying a background image to the containing div that wraps both the content and sidebar areas.
Here’s a good article:
http://line25.com/articles/create-sidebars-of-equal-height-with-faux-columns