As of the launch, the Sunday Keys App requires less than one gigabyte of storage space. This includes both the App download itself and the Sunday Keys Sound Library, which is downloaded once you login to your account in the App.
Since the Sunday Keys Sound Library will continue to be expanded, users should expect the long-term storage space required to be a good bit higher, likely in the range of 5 to 10 GB of space within the next year.
As additional Sunday Sounds products are made available within the App in the future, each downloaded product will require some additional storage space. You'll always be able to see how much space is required for a given product within Menu>Check for Downloads in the App.