Being a widescreen user, the narrow width of the WordPress Codex drives me nuts. It literally uses half the width of my screen. So, I threw together a couple lines of CSS a year or two back and added them to Stylish to make the site wider as well as to remove those “Edit” links that show up next to headers when logged in (I don’t like them). Please mind you I’m not a designer and there’s probably a better way to do this (suggestions welcome), but it works (at least for me), so whatever. 🙂

@namespace url('http://www.w3.org/1999/xhtml');

@-moz-document domain('codex.wordpress.org') {
.wrapper { width: 1400px !important; margin-left: auto; margin-right: auto; }
.col-10 { width: 1200px !important; } 
.editsection, .toctoggle { display: none !important; }
}

Note you will need to change those widths a bit as I couldn’t get it working properly using a flexible width (due to the sidebar)’