Change thumbnail size

Is there a way of increasing thumbnail size for videos?

I’m ok with getting plugin files with FTP and changing a couple of lines of code. But I need some guidance on this. I am not planning to update wordpress/plugin afterwards as I know that all changes will be lost.

Also, I just started using the plugin, so re-uploading videos in order to regenerate thumbnails is not a problem for me.

https://www.ads-software.com/plugins/workbox-video-from-vimeo-youtube-plugin/