gmail-navbar-width.user.js

// ==UserScript==
// @name        Gmail Navbar Width
// @namespace   http://tampermonkey.net/
// @match       https://mail.google.com/mail/u/*
// @description Make the gmail navbar on the left a bit smaller
// @version     0.0.4
// @homepageURL https://gist.github.com/gurgeous/98c38b591151a83145a9c55a6629d752
// @updateURL   https://gist.github.com/gurgeous/98c38b591151a83145a9c55a6629d752/raw/gmail-navbar-width.user.js
// @downloadURL https://gist.github.com/gurgeous/98c38b591151a83145a9c55a6629d752/raw/gmail-navbar-width.user.js
// ==/UserScript==

// the "div" is important for specificity
const $style = document.createElement("style");
$style.textContent = `
  /* left navbar width */
  div.aeN {
    min-width: 190px !important;
    max-width: 190px !important;
  }

  /* and the interior bits, to show numbers */
  div.wT {
    min-width: 174px !important;
    max-width: 174px !important;
  }
`;
document.querySelector("head").appendChild($style);