It's there, in two parts.
First, in tab 'Layout' set your total page width.
Second, in tab 'Sidebars' set your 'Left Sidebar Width' and your 'Right Sidebar Width'
The centre column width will always be total width minus the sum of the sidebar widths.
On any pages or posts that you set not to display the right and/or left sidebars, the centre column will expand to take up the width of the removed sidebar(s).
If you are using left and right opacity on the header image set these widths to match the corresponding sidebar widths to give a consistent look to your site.