Dear Kevin,
After reading the link you posted and testing, all you need to do is add one of 'vjs-fluid, vjs-16-9, or vjs-4-3' to the core 'media_videojs | videocssclass' setting, i.e. Site administration -> Plugins -> Media players -> VideoJS player. So that setting could contain 'video-js vjs-fluid' as an example.
After reading the link you posted and testing, all you need to do is add one of 'vjs-fluid, vjs-16-9, or vjs-4-3' to the core 'media_videojs | videocssclass' setting, i.e. Site administration -> Plugins -> Media players -> VideoJS player. So that setting could contain 'video-js vjs-fluid' as an example.
But if you want 'fill' instead of 'fluid' then use the class 'vjs-fill' instead, so 'video-js vjs-fill' instead.
Gareth
Gareth