Clone this bug from bug 12188.
archive.ar_len should be populated on all Wikimedia wikis. If for no other reason than it causes inconsistencies (null entries) in revision.rev_len when revisions are restored (really a separate bug, but let's just populate everything anyway and be done with it).
This bug may require a separate bug to actually add this functionality to MediaWiki core's maintenance scripts. I don't think there's a script that does this currently (populateRevisionLength.php could have an --archive flag, maybe). But this bug is for populating archive.ar_len on all Wikimedia wikis.