Resize WP Video Lightbox Thumbnail Image


I'm using the WP Video Lightbox plugin on a site and would like to increase the size of the video thumbnails. See example on this page:

I tried this CSS, but it did not work:
.video_lightbox_auto_anchor_image {
width: 600px !important;
height: 450px; !important;}

Please advise.

Thanks in advance,