I took a screenshot of my app on iPhone6. However, the size for this screenshot is 640 * 1136. According to documents for apple, the size for iPhone6 screenshot should be 750 * 1334. I just cannot figure out the reason for the wrong screenshot size.
See this link: http://www.paintcodeapp.com/news/ultimate-guide-to-iphone-resolutions
You can change display zoom at Settings