By default, WordPress has auto embed feature. It means, if you paste any Youtube video or Flickr image URL in the post – it will automatically show up (respective Youtube video player or Flickr image). Width of displayed content depends on size setting you have specified for maximum width of photo or video player you are trying to display.
You can easily change and adjust width (or height) of the content that is displayed by auto embed method in a WordPress blog. Goto Settings > Media in your WordPress Dashboard and change width – height values next “Maximum embed size” under “Embeds” section.
Make sure “When possible, embed the media content from a URL directly onto the page. For example: links to Flickr and YouTube” option is checked for auto embed feature to work and provided you are not using any special plugin to display Youtube videos and flickr images on your blog.