1920x720?
???
Sure you don't mean 1920x1080, or 1280x720? Because if your source frame size really is 1920x720 (2.66:1???), then 853x480 (16:9, the standard HDTV aspect ratio) would give you the wrong aspect ratio.
Twenty minutes isn't too bad, and the small-ish 853x480 frame size actually gives you breathing space. But you have to pick the right wrapper, the right codec, and the right bit rate. Also sacrifice a bit of data rate on the audio; it will make more difference in the picture. Try MPEG-4 in H.264 codec, 3000-5000 kbps. (You can try higher than 5000kbps, but in my own experience, that's usually around the threshold when people start having problems streaming the clip, even after YouTube's Flash compression)
If picture quality is your concern over the amount of time it takes to compress, then in whatever software you're using (Compressor? MPEG Streamclip? Sorenson Squeeze?), activate the multi-pass and b-frame options.
People always say, "Apple gets this great quality for this little file size". Well, the studios pay professional rates at professional facilities to get those results. Do you seriously think you can get the same results with your $10,000 computer/software as a $50 million production can?
www.derekmok.com