Check out this infographic: http://www.paintcodeapp.com/news/iphone-6-screens-demystified
It explains the differences between old iPhones, iPhone 6 and iPhone 6 Plus. You can see comparison of screen sizes in points, rendered pixels and physical pixels. You will also find answer to your question there:
iPhone 6 Plus - with Retina display HD. Scaling factor is 3 and the image is afterwards downscaled from rendered 2208 × 1242 pixels to 1920 × 1080 pixels.
The downscaling ratio is 1920 / 2208 = 1080 / 1242 = 20 / 23. That means every 23 pixels from the original render have to be mapped to 20 physical pixels. In other words the image is scaled down to approximately 87% of its original size.
Update:
There is an updated version of infographic mentioned above. It contains more detailed info about screen resolution differences and it covers all iPhone models so far, including 4 inch devices.
http://www.paintcodeapp.com/news/ultimate-guide-to-iphone-resolutions