Autoblog YouTube

Hi
I need to control the output size of the video.
Currently I get a 900x506 video ..thus killing the layout of site
please explain hoe and exactly where to limit or fix the output size of the youtube embed
thank you
please see for example
http://ningboexpat.com/my-125-gallon-reef/

code: <p><iframe width="900" height="506" src="http://www.youtube.com/embed/uepuFYCKw0c?fs=1&feature=oembed" frameborder="0" allowfullscreen></iframe></p>
Eckhard