You should use the API as Nils suggested for one check or an external application. But if you are needing the DB the file sizes for reporting of the attachments they are in jiveattachment (not sure if that is maintained when using an external storage provider like Google and this data may also be available in the data export api). I've often used the following SQL to summarize the entire size of user contributed files.
select count(attachmentid), sum(filesize), contenttype from jiveattachment group by contenttype order by count(attachmentid);
Keep in mind this might not be 100% accurate since file size here is actual and disk space is a different thing (overhead etc.). But it is a nice summary of your attachments by content type.