Add this to the functions.php

/**
 * Add Response code to video embeds in WordPress
 *
 * @refer  http://alxmedia.se/code/2013/10/make-wordpress-default-video-embeds-responsive/
 */
function abl1035_alx_embed_html( $html ) {
  
  return '<div class="video-container">' . $html . '</div>';
}
add_filter( 'embed_oembed_html', 'abl1035_alx_embed_html', 10, 3 );
add_filter( 'video_embed_html', 'abl1035_alx_embed_html' ); // Jetpack

Add this to your stylesheet

/*---------- Video Embeds ---------*/
  
.video-container { 
  position: relative; 
  padding-bottom: 56.25%; 
  height: 0; 
  overflow: hidden;
  max-width: 1200px;
  margin: 0 auto;
}
  
.video-container iframe, .video-container object, .video-container embed, .video-container video { 
  position: absolute; 
  top: 0; 
  left: 0; 
  right: 0;
  width: 100%;
  height: 100%;
}

Adjust or remove max-width as needed.