In WordPress 3.5, there’s no more option to set the width and height of oEmbed-embedded media (Youtube, Vimeo, Flickr, Twitter etc) in the admin interface. The max width is now automatically the width defined in the functions.php of your theme:

if ( ! isset( $content_width ) ) $content_width = 600;

Just change 600 in the above code to whatever the width of your main content area is in pixels. More info about the change on WP Beginner.