As mentioned before, every iCloud user gets 5 GB of storage for free. You can see how much storage you've used, what data is stored on iCloud, and also upgrade your current storage plan to a higher one from the iCloud preference pane on Mac, iCloud settings on iOS, or iCloud Control Panel on Windows PC. You're also allowed to downgrade your current storage plan if you've upgraded it before.
In iOS, you can manage your iCloud storage and see the list of all documents that are already uploaded to iCloud. You can also delete any iOS device's backup or documents that you want to. The following instructions show you how to do this: