change the default Video ratio

I created a child theme and I would like the default setting to change for the video ratio. Currently the dimension is [video width="1280" height="720" . Can I use a plugin within the wpmudev to change the ratio I choose or is there code that I add to a file so the default ratio will be set to a different video dimension such as [video width="180" height="120"

or

would anyone know if I could possible change this in style.css file and how would this be done?