What does it look like when an Instagram user using an iPhone X comes across an iPhone X size image in Stories?
Instagram limits the sizes of the photos that can appear in their stories. Even though you upload for example a high resolution screenshot from an iPhone X, it will be scaled to a lower resolution and even cropped to fit their choice of aspect ratio.

Instagram currently allows for square images (up to 1080x1080 px), vertical images (up to 1080x1350 px) and horizontal images (up to 1080x566px).

It doesn't matter if you're uploading/viewing this on an iPhone X, the same restrictions apply.