Replying to Gopro and Imovie Help.

All my gopro footage is shot in 1080p hd. The problem is when I input it into imovie it automatically changes the dimensions. I click the "full size" box when adding the footage to imovie and it still shrinks it. The main problem with this is it doesn't allow me to upload a 1080p video to youtube. The best quality I can get is 480p on youtube. If I upload the footage without putting it into imovie I can get 1080p quality on youtube with no problem. Anyone know how to upload 1080p videos to youtube after editing them in imovie? Thanks.