How to upload an iMovie to Youtube in 1080p?

I made a little clip with iMovie in 1080p (recorded it with a Nikon D5100) and I can't seem to upload it to Youtube with 1080p. I used the Share button iMovie has and clicked on Youtube and uploaded it from there but it uploaded it on 720p. Then I extracted the video and saved it to my computer and uploaded it from Youtube but it uploaded it 360p! So how am I supposed to upload it in 1080p?

Export it in 1080p to your documents then directly upload it to youtube.