Trouble Resizing Videos

I am using this theme and finding that I can’t resize YouTube videos either using <iframe> code or WP’s [embed] code. It seems the width of the video always goes to the width of the entry-content class.

Unfortunately, this site is behind a firewall so I can’t show you the problem. But, I’ve tried all sorts of plugins (YouTube plugin for instance plus using the iframe and embed as I mentioned above) all to no avail.

Do you have any reason why I may be having this problem? Thx.