http://revolution.themepunch.com/documentation/#youtube-vimeo

I think 1920X1080 should be fine. Here is how I plan to display your video. I used a random youtube video. https://ashlandwebsites.com/video-background-test/

The video I used is 1080p: 1920×1080 but it is set to automatically display at 720p: 1280×720. Hover the video and click the cog to change the quality. You can see how it looks at 1080.

You can set the auto display quality on youtube. See here: https://support.google.com/youtube/answer/91449?hl=en

If you want to create the video large that is fine too. You can always make it smaller but not bigger so better to start big. The only thing is, I am not sure if I movie exports larger than 1080. That’s something you might look into with imovie.