What is the end result that you are trying to achieve? Do you want the left and right columns to take up a specific percentage of the width (e.g. 50/50, 60/40, etc.)
Or do you want the left column to be a fixed width and have the right column take up the remainder of the page?
Is this a web part page, or a wiki page? If it is a web part page, which template did you select when you created the page?