Updated Procedure: https://www.nowherelan.com/2018/12/26/expand-the-main-content-page-on-the-wordpress-twenty-fourteen-theme/

To expand the width of the main content page when using the WordPress 3.8 Twenty Fourteen theme:

• Login to WordPress as an administrator
• Go to Appearance > Widgets
• Remove all widgets from the Content Sidebar
• Go to Appearance > Editor > style.css
• Find the section that says:

.page-content {
 margin: 0 auto;
 max-width: 474px;
}

• Change it to:

.page-content {
 margin: 0 auto;
 max-width: 990px;
}

• Click Update File

My System Configuration

• WordPress 3.8
• Twenty Fourteen theme 1.0

References

• http://en.forums.wordpress.com/topic/twenty-fourteen-theme-css-customization
• http://core.trac.wordpress.org/ticket/25031