Is mbox size field actually needed?

The mbox size field is populated by the archiver, but not by migrate.py.
It is only ever read by rethread.py, and then only checked for presence.

It does not seem to serve any purpose currently.

Would it ever be needed?
Would it ever be useful as part of a query?

At the very least the purpose of the field needs to be documented.

Trace the mbox size field through the archiver, migrate.py, and rethread.py, noting where it is populated, read, and checked for presence. Determine whether it has a current purpose or query value; done means the field's role is documented, or the issue records a clear basis for changing it.

Written by the indexing model from the issue text.