In version 5 of videojs you can use vjs-16-9 vjs-4-3 class on video object,
<video class="video-js vjs-default-skin vjs-16-9" ...>
...
</video>
or use fluid option
<video class="video-js vjs-default-skin" data-setup='{"fluid": true}' ...>
...
</video>
Source: https://coolestguidesontheplanet.com/videodrome/videojs/