That's a standard problem / complaint in design as there isn't currently a great way to do this.

The easiest way is to use a full-width background image behind the content <div> that's tiled vertically that has brown on the left (under where the sidebar would go) and white on the main content, so that even when the sidebar is short it still appears to be full length.

Your other option is to add height to the sidebar class - so in your stylesheet look for: #leftcol and add a new line with either:
min-height:600px;
or
height:600px;

Replace the 600 with the height you'd like. Difference is that min-height can expand beyond 600px, whereas height locks the height at 600px.

I've always gone into my body tag and added
style="height:100%;"

Is that method outdated now-a-days? Meh.