Youtube width too big

Hi. I have a blog using theme arclite. The problem is when I embed videos on youtube, if the width is too big, the youtube video goes in back of the theme. I mean:
– the video should look like this:http://img121.imageshack.us/img121/5033/test1xh.jpg

and not like this:http://img810.imageshack.us/img810/3185/test2c.jpg
In the second picture the video goes in background and not all shows up. How can I fix that? I think it should be something in CSS but i don’t know what. Thanks in advance.