There is no "supported" method for this. You could probably write a script to check the ZODB repository, but that would be tricky. If you want to find the largest files on your Plone site, you are probably better off writing a script that will work in Plone and use it to search (using portal_catalog) for all File objects (or any other type of content that is most likely have large files) and call get_size () on it. This should return the size (cached) and you can delete what you want to clear.
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