To enable the resizable YouTube player, go to any video on YouTube. Then, open the developer console (Ctrl-Shift-J in Chrome and Opera, Ctrl-Shift-K in Firefox). In the console, paste the following code:
document.cookie="VISITOR_INFO1_LIVE=E_OLzg3yeLw; path=/; domain=.youtube.com";window.location.reload();
This will enable the experiment and reload the page. Now the player should resize with the window. As always, it should be noted that YouTube experiments can change at any time and without warning so there’s no guarantee this will always work. Enjoy it while it does.