The bigger the phone, the harder it is to type with one hand. While Apple's Reachability feature certainly alleviates some of the pain in transitioning to a larger phone, it doesn't help when it comes to entering text on the larger keyboards.
That's where the aptly titled app, One Handed Keyboard comes in. It displays what it claims to be the same size keyboard iPhone users had grown accustomed to over the years.
Currently, the app offers only a smaller keyboard that docks to either side of the screen, with extra controls to adjust the theme or color of the keys.
Unfortunately, there's no auto-correct library, meaning that whatever you type is what's displayed, with no way for iOS to correct it. However, the App Store listing does mention this as a feature coming in an update "soon."
Should you decide to purchase the app, you'll need to activate the keyboard in the iOS Settings app by going to General >Keyboard > Keyboards > Add New Keyboard > One Handed Keyboard.
More detailed instructions on activating third-party iOS 8 keyboards can be found in this post.