Export settings for audiobooks
If your audiobook is audio only, and there is only a single image associated with it, I would simply distribute it as audio with the image. 

If you want to upload to YouTube or another streaming distribution site, use the preset meant for YouTube, or specify what site.

For a technical answer to your question assuming you're using H.264 or H.265; make the keyframe distance as long as possible, use 2 pass encoding, and use the native resolution and frame rate of your distribution medium.