Different youtube embed size in ephemera widget and video post format

Hi!
I’m building a child theme based on twentyfourteen, and it’s nearly finished. The whole idea for the theme is to have really big photos and videos, if either the post has post-format “video” or “image”. So far it works wonderful!

Today, however, I noticed that the ephemera widget displays videos at the same size I’ve applied in the post, and that’s no good.

I want the widget video to be the same size as it was before, but the video in the post (post-format = video) itself should have a huge video (1260x709px).

Does anybody know the css to fix this?