Simple User script that gives the dimensions of the current window.

window.dimensions.user.js

// ==UserScript==
// @name         Window Dimensions
// @namespace    http://stripedpurple.io
// @version      2.0
// @description  adds a dimension list to screen
// @author       Austin Barrett
// @match         *://localhost/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    var createDimensions = function() {
        var dimension = document.createElement("span"); // Create a <button> element
        dimension.setAttribute("id", "myDimensions");

        var t = document.createTextNode((function() {
            var w = window.innerWidth;
            var h = window.innerHeight;
            return w + ' x ' + h;
        })());

        dimension.appendChild(t);
        dimension.style.background = "#333";
        dimension.style.color = "white";
        dimension.style.position = "fixed";
        dimension.style.bottom = "0";
        dimension.style.right = "0";
        dimension.style.zIndex = "100000";
        dimension.style.padding = "3px";
        dimension.style.fontSize = "12px";
        dimension.style.fontFamily = "Arial";


        document.body.appendChild(dimension);
    };

    var updateDimensions = function() {
            var w = window.innerWidth;
            var h = window.innerHeight;
            document.getElementById("myDimensions").innerHTML = w + ' x ' + h;
    };

    window.onload = createDimensions;
    window.onresize = updateDimensions;
})();