you've already got sweet 1080p on youtube but it also goes through their compression algorithms there
when i first started putting videos on the website, i chose 1136x639 because it was the right size to fit comfortably on the screen of my default web browser on my screen, without zooming out. it was very uncomfortable to load a video which by default goes off the edge of the screen, and the browser(s) i was using did not automatically shrink it to fullscreen. actually they still don't, so i continue to do that. also slower resolutions load faster so sometimes i think i should make it even lower