When you say "bringing in a file (no larger than 2000x1333 px ",you're talking about a photo, right?
The resolution of the photo you add to your video project doesn't affect the final appearance of the video a whole lot. Although some people have found that the closer the photo is to the final resolution of the video, the better looking your results. This is because you can do a better job of resizing your photos (using the bicubic sharpener) than the program does. The program just throws away pixels. If you use the bicubic sharpener when you reduce the size of your photos, you create a sharper photo. Make sense?
The best format and resolution to output your video-for-web at depends on where you're going to post the file. If you're posting to a site like Vimeo or YouTube, you can post a 1920x1080 AVCHD and the site will re-encode and optimize the file for web streaming.
However, if you're posting video to your own personal web site, you need to consider things like the best format and resolution for streaming your video. If you post a 1920x1080 video, it may look great -- but it may stream so slowly that no one will be able to watch it online.
Hey, many thanks again Steve. That’s great. The few I’ve posted online up to now have all been on You Tube.
The ideal format to post to YouTube is found under Publish & Share/Computer/AVCHD. Under the drop-down menu you should find a preset for YouTube HD.