Only options I see are 1920X1280 or 1620X1080
The exact settings depend on how you're going to use the file once you output it. Do you plan to use it as source footage in a new project? To create a DVD or BluRay? To display your project online? To share your project to a smartphone or iPad?
But basically, you use Publish & Share/Computer/AVCHD with the appropriate preset.
Although this will technically produce an MP4 file, this file is a Quicktime-structured file. In fact, if you feel the need, you can change the suffix to .mov and it will still play. Though you don't need to.