If you go into the HTML source, you shoud see an iframe tag as shown below. Change the width property in this tag.
<iframe width="420" height="315" src="//www.youtube.com/embed/MVSs4-9Ujgg" frameborder="0" allowfullscreen></iframe>
For custom css, you could wrap the iframe in a div, and then add inline styling to that div. For example, this would add a 1 pixel black border around the video:
<div style="border: 1px black solid;">
<iframe width="420" height="315" src="//www.youtube.com/embed/MVSs4-9Ujgg" frameborder="0" allowfullscreen></iframe>
</div>