Here’s a small jQuery code snippet that I found on Anders Noréen‘s themes that makes sure embeded videos have the correct size ratio on any screen.

(function() {
    // resize videos after container
	var vidSelector = "iframe, object, video";
	var resizeVideo = function(sSel) {
		$( sSel ).each(function() {
			var $video = $(this),
				$container = $video.parent(),
				iTargetWidth = $container.width();

			if ( !$video.attr("data-origwidth") ) {
				$video.attr("data-origwidth", $video.attr("width"));
				$video.attr("data-origheight", $video.attr("height"));
			}

			var ratio = iTargetWidth / $video.attr("data-origwidth");

			$video.css("width", iTargetWidth + "px");
			$video.css("height", ( $video.attr("data-origheight") * ratio ) + "px");
		});
	};

	resizeVideo(vidSelector);

	$(window).resize(function() {
		resizeVideo(vidSelector);
	});
})();