As of today, the latest whatsapp-web update makes this extension useless, since meta devs "fixed" the fullscreen display of the site (finally).

Fullscreen WhatsApp Web

CSS extension that aims to improve the whatsapp web experience by displaying the app content in fullscreen, also lets you resize the chats-section sidebar.
_{Credits to @shreyash-b (and other contribs) for the original code}

Installation Instructions:

• Download this repo and extract it somewhere safe
• On your browser, navigate to chrome://extensions and enable Developer Mode there
• Click Load Unpacked and select the folder that contains the extension files (Fullscreen-Whatsapp-Web-main)
• That's it, remember not to delete the downloaded folder, otherwise the extension goes as well!

Before:

After: