Define a max size for storage

I know that several participants have mentioned that they are uncomfortable with storage requirements, but I didn't see anything related specifically to this:

There is no defined maximum size (limit) for the amount of storage that a UA should allocate for reports that are not yet submitted. This seems like a potential security violation in the case where:

1. A report end point is invalid (therefore the report is stored for repeated retry)
2. Reports are generated rapidly (therefore storage use increases rapidly)

I hope that this is useful feedback. Please ignore/close if this is not important.