Needs the following JS script code (installed with GreaseMonkey) to resize the album art since the page will (re-)inject an <A> element with hardcoded size when the image is replaced.

// ==UserScript==
// @name		Spotify Play - bigger cover art
// @namespace	        http://njus.no
// @version		1.0.0
// @include		https://play.spotify.com/*
// @copyright           2015, Kjetil Torgrim Homme
// @description         Override album art size
// ==/UserScript==

setInterval(function ()
    {
        // See if we have a player, and adjust size every 2 seconds
        var pl = document.getElementById('app-player');
        if (pl) {
            var art = pl.contentDocument.getElementById('cover-art').getElementsByTagName('a')[0];
            if (art) {
                art.setAttribute('style', "width: 300px; height: 300px;");
            }
        }
    }, 2000);