Topic: Best Youtube video size?

I am creating a soundslide project to convert to video.  It looks like for posting on Vimeo 1280x720 is the ideal size.   My question:  is the same also true for Youtube?  I am finding conflicting information on the web.  I can produce multiple versions -- no problem -- I just want to make sure I'm close to 100% so that there's a minimum of downsizes or upsizes in Youtube's processing.

Thank you!