The short answer:
Anything higher than 1500px wide. Narrative looks after the rest.
The long answer
When you publish your images to Narrative, your images will be uploaded to the Narrative server and automatically resized to 5 different image sizes to ensure your website runs fast on all screen sizes.
The images sizes that Narrative uses are:
Will I lose image quality when my images are resized?
No, not noticeable to the eye. In fact, expect your images to look better when resized with Narrative. Images that are scaled from a larger size to fit a small screen often lose sharpness. Narrative images always look great.