For bulk file work, avg about 8 Mb which is not that big. For backup work, tar/rsync/scp etc up to 600Gb per file. Biggest "transactions are however database work. Up to 80_000_000 record traversals per process several times a day.

Linux (ext3 and ext4), HP-UX (jfs) and AIX (jfs) and several NFS based processes (small files).

Data files are mostly plain CSV or CSV file(s) inside a ZIP. I'm not doing a lot of XML. Binary, HTML and other formats occasionally. CSV is not-fixed length, but some binary files are (though we are pushing the organizations that give us those to move to CSV/UTF-8).