We have two video plugins, Video Showcase and VidEmbed, neither of which are technically responsive at this time. We have recently released Builder Responsive and are now working to make our plugins responsive as well. Both support S3 and self-hosted videos, however.
With Video Showcase, you select the size of your video's thumbnails. While these don't resize, you can see that they reorganize to fit in the width:
The pop up window that comes up when the video is clicked also fits the screen.
VidEmbed simply places the actual video on the page. It uses the player from the source, so if, for instance, you use a video from YouTube, it will resize if the player resizes.
Let us know if you have additional questions.