Open /blogs/plugins/videoplugin/_video.plugin.php and change the size in:

$content = preg_replace( '¤\[video:youtube:(.+?)]¤', '<div class="videoblock">
<object data="http://www.youtube.com/v/\\1" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350">
<param name="movie" value="http://www.youtube.com/v/\\1"></param><param name="wmode" value="transparent">
</param></object></div>', $content );

Good luck

And that will only change the size in the side bar preview and not in my actual blog, right?

Changing here will change in all places where you use the video plugin. You may want a duplicate entry for YouTube with different sizes.

Good luck

Thanks appreciate it. =]