Get the size of the screen, current web page and browser window

This has everything you need to know: Get viewport/window size

but in short:

var win = window,
    doc = document,
    docElem = doc.documentElement,
    body = doc.getElementsByTagName('body')[0],
    x = win.innerWidth || docElem.clientWidth || body.clientWidth,
    y = win.innerHeight|| docElem.clientHeight|| body.clientHeight;
alert(x + ' × ' + y);

Fiddle

Please stop editing this answer. It's been edited 22 times now by different people to match their code format preference. It's also been pointed out that this isn't required if you only want to target modern browsers - if so you only need the following:

const width  = window.innerWidth || document.documentElement.clientWidth || 
document.body.clientWidth;
const height = window.innerHeight|| document.documentElement.clientHeight|| 
document.body.clientHeight;

console.log(width, height);

You can get the size of the window or document with jQuery:

// Size of browser viewport.
$(window).height();
$(window).width();

// Size of HTML document (same as pageHeight/pageWidth in screenshot).
$(document).height();
$(document).width();

For screen size you can use the screen object:

window.screen.height;
window.screen.width;

A non-jQuery way to get the available screen dimension. window.screen.width/height has already been put up, but for responsive webdesign and completeness sake I think its worth to mention those attributes:

alert(window.screen.availWidth);
alert(window.screen.availHeight);

http://www.quirksmode.org/dom/w3c_cssom.html#t10 :

availWidth and availHeight - The available width and height on the screen (excluding OS taskbars and such).

Here is a cross browser solution with pure JavaScript (Source):

var width = window.innerWidth
|| document.documentElement.clientWidth
|| document.body.clientWidth;

var height = window.innerHeight
|| document.documentElement.clientHeight
|| document.body.clientHeight;