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 via Narrative's publishing service and automatically resized to 5 different image sizes to ensure your website runs fast on all screen sizes.
By doing this, we can ensure that your post loads fast on all devices and screen sizes. For example. If someone views your blog on the iPhone (with a screen size of 500 pixels wide), Narrative will deliver the image that is only 500 pixels wide. This is important because it saves precious bandwidth and therefore ensures the post loads fast on every user's device.
The images sizes that Narrative uses are:
1500px
1000px
750px
500px
300px
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.