ps-enlarge-battle.user.js

// ==UserScript==
// @name         Enlarge Pokemon Showdown Battle
// @match        https://play.pokemonshowdown.com/*
// ==/UserScript==

(function() {
    'use strict';

    const style = document.createElement('style');
    const scale = 1.5;
    style.textContent += `.battle, .battle-controls { transform: scale(${scale}) !important; transform-origin: top left; }`;
    style.textContent += `.battle-controls { top: calc(370px * ${scale}) !important; }`;
    style.textContent += `.innerbattle { width: 100%; height: 100%; }`;
    style.textContent += `.battle-userlist, .battle-log, .battle-log-add { left: calc(640px * ${scale}) !important; }`;
    document.head.append(style);
})();