Maybe once a month open the app editor and check it if the number of clients is not so large that the process is too cumbersome.
I’d also check to see if there is any exposure of this information via the API, like maybe the meta api call would expose this information. If not, potential post it as an idea.
Unfortunately,
while it is possible to check the file storage of an app in Bubble via the Setting tab → App Plan → Storage,
Bubble doesn’t offer this as a dynamic value that can be referenced to display to users.