yes it is possible... first goto,
ACP >> Templates >> Your theme's Templates >> Video MyCode Templates >> video_youtube_embed
the width and height of the video is defined within the iframe and hence it remians constant... lets first remove those width and height add a css class instead...
change this....
<iframe width="560" height="315" src="http://www.youtube.com/embed/{$id}" frameborder="0" allowfullscreen></iframe>
to this....
<iframe class="yt" src="http://www.youtube.com/embed/{$id}" frameborder="0" allowfullscreen></iframe>
then goto global.css and these codes at the bottom...
@media screen and (max-width: 960px) {
iframe.yt {
width: 100%;
}
}
iframe.yt {
width: 560px;
height: 315px;
}
now, adjust the max-width as per your wish... save everything, open your forum and hard refresh it (CTRL+F5)...