First question... are you setting your export to go on a DVD or a BluRay?
I was trying to export to a file. I tried multiple formats. Apparently, you no longer select the desired output when you create a project and Premiere based my outpu size on the first file I inserted. I am currently recreating the whole project, MASSIVE PAIN IN THE BUTT, but I inserted the HD video clips first and everthing seems to be going good now. I even did a test export to make sure. Lesson learned, start with the HD and work your way down.