There are features that help to operate a large phone with one hand like reachability for iOS or one-handed mode for Samsung Galaxy Note. Is there such an ability for Windows Phone 8.1 (Nokia Lumia 1520)?
There are a few accessibility features on Windows Phone you can find in your settings, but none that specifically help you operate large screen phones. You might want to check out http://windowsphone.uservoice.com and add this as a suggestion.
EDIT: Microsoft announced that there will be a feature in Windows 10 Mobile just like in iOS. It is already part of the latest preview build and works on all devices with 5" screens or larger.