Hello @Frank5! These are great questions. The storage limitation does not reset every month. This is your allocated storage for your whole community during its existence.
Currently, we have an option to disable the option to upload files when posting a content but it is not yet possible to set a file size limit on what can be uploaded by a member or to automatically delete large files. By default, the file limit size is 500 MB. I will submit a suggestion to our team about this including having an option to monitor the files that are uploaded.