This code helps you set up a video tag that will adapt its size to its container's.

<video preload="auto" controls="controls" style="width: 100%; max-width: 100%; height: auto;">
    <source src="media.mp4" />
    <source src="media.ogv" />
    <object width="640" height="480" type="application/x-shockwave-flash" data="media.swf">
        <param name="movie" value="media.swf" />
        <param name="wmode" value="transparent" />
        <img src="media.jpg" alt="Y U NO READ ANYTHING!?" />
    </object>
</video>