+make-gmail-font-size-larger.user.js: Firefox GreaseMonkey or Chrome Tampermonkey script: Make Gmail font size larger because it sucks by default.

+make-gmail-font-size-larger.user.js

// ==UserScript==
// @name        Gmail Font Size Changer
// @namespace   http://mky.io
// @include     https://mail.google.com/mail*
// @description Make Gmail font size larger because it sucks by default.
// @grant       none
// @version     201607282
// ==/UserScript==

(function() {
    'use strict';
    function addGlobalStyle(css) {
        var head, style;
        head = document.getElementsByTagName('head')[0];
        if (!head) { return; }
        style = document.createElement('style');
        style.type = 'text/css';
        style.innerHTML = css;
        head.appendChild(style);
    }
    addGlobalStyle('body { font-size: 20px; line-height: 1.6; }');
})();