This tutorial explains how you can set the template width and the width of the left, right, inner left and inner right column in your YOOtheme template. The following illustration shows the name of the module positions that belong to the columns and a screenshot from the template configuration where you can edit their widths.
When editing these values it is likely that you have to edit background images which, in many YOOtheme templates, define the inner borders of your columns. Usually, you only have to add the amount of pixels that you added to your column to your background image, too.