At what resolution will an iPhone only app run at on an iPad Pro?
375x667pts or 4.7" (iPhone 6 screen)
When running and iPhone only app in the iPad Pro simulator in Xcode 7.1 (publicly available in the App Store) the screen point resolution is reported as 375x667 or a 4.7" screen. This also equates to an iPhone 6 or an iPhone 6+ in display zoom mode. See PaintCode's guide for more details on display zoom mode.
- http://www.paintcodeapp.com/news/ultimate-guide-to-iphone-resolutions