You can't save any data before the app Launches for the first time.i.e. If user user just install the app & not open the app then Document Directory must be blank.
If you launches the app then you can save data as much big as memory is free in the device.
If you store files in Documents, they will be backed up to iCloud automatically. If you don't want that, the easiest way is to just store them somewhere else. Library/Caches is a good choice. If you really MUST store in Documents but you want to disable iCloud backup you can use the technique outlined here:
https://developer.apple.com/library/ios/#qa/qa1719/_index.html