I’ll add a comment to this: simply put, do the designs in 320 x 568 because that’s the natural resolution of iOS. Export at 2x and it will be as if you did it at 640 x 1136.
Doubling the natural resolution is a Photoshop way of doing it; Sketch is more like the web design world. My breakpoints don’t think that the iPhone is 640 x 1136; otherwise my web designs would look stupid. My code thinks it’s only 320 x 568. And that’s how Sketch operates too.
2x and 3x export exists for Retina devices.