I'm designing a keyboard that takes advantage of the larger screen of iPhone 6 Plus (but on iPhone 6 it's usable to some extent too). My keyboard is basically useless on pre-4.7 inch devices (e.g. iPhone 5s and below) and for this reason I need to target only iPhone 6, iPhone 6 Plus, and any future iPhones. Is this possible? If yes, how? If not, what would be a good workaround?
You cannot submit an app to the app store that exclusively runs on iPhone 6. You must also support 5, 5c, 5s and 4s. There is no workaround other than providing a version of your keyboard that works on all of these devices.