Using a program such as adobe premiere create a project with the video dimensions 1280x720 and then put your raw video in there. When uploaded to youtube it will remain in HD.
I don't know of any where you can select the video canvas size. Personally I use Adobe Premiere Pro. If you only want to use it for one video you could just download the 30 day trial from www.adobe.com. it doesn't leave any watermarks or anything.