If you have a Wikipedia account, you can go to Preferences > Appearance[0], and choose a different skin. AFAICT "Monobook" and "modern" have unlimited content width.

If you don't then you can append ?useskin=modern to each article URL. (You can do it automatically with a redirect extension.)

Alternatively you can inject the following CSS (e.g. with Stylus):

    .mw-content-container {
        max-width: none !important;
    }

(The specific CSS selector might change with time.)

[0] https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsec...