Note: Insert a dash between the yout and ube of youtube.com to get a full-window, ad-free vid.
or for ease of use, create a bookmarklet on the bookmark bar,
Name: See Vids NoAds
Address: javascript:(function(){window.open(location.href.replace('youtube.com/','yout-ube.com/'), '_blank').focus();})();
From then on, from any youtube page, pause the vid, and click that new bookmarklet to view that vid full-window.