The inserted video is an iframe tag and is given the CSS class of 'ql-video' so you can customize that class for your needs.
I added a little repo to allow video resize. I just started messing Quill this week so there may be better ways to do it, like make it a module. But it works so that's nice.
https://github.com/lancetipton04/quill-video-resize
@listar do you have the installation steps? Is this on NPM?
@lancetipton I couldn't get your video-resize to work. It gives me an error saying "e is not a constructor" while running quill's theme class on your module. I've used the default settings for videoresize module