How do I save a story as an MP4 in 16:9 resolution?

I can adjust the dimensions of the story to whatever I like, which is good. I need the story to be in 16:9, so that's how I create it. However, when I go to save the file it only gives me three resolution options:
320x240
480x360
640x480
These resolutions all create black side bars on either side of the story. I need to save this as an MP4 file, and it needs to fit into a 16:9 resolution without the black side bars.