Thankfully the MO3 unpacker by Ian Luck (MO3 author) is fairly forgiving when processing a MO3 with incorrect file size, it just unpacks it without any issues so I had no extra work to figure out the original file size of the MO3 modules.
Just a note, if you unpack MO3s for permanent distribution, please use the latest "stuff" version from
this thread - I don't think the original MO3 package has been updated yet, so it might still contain a version of UNMO3 which writes out malformed IT files (if the source format was IT).