Currently, postconf.5 has this to say about message_size_limit:
message_size_limit (default: 10240000)
The maximal size in bytes of a message, including envelope information.
Note: be careful when making changes. Excessively small values will result
in the loss of non-delivery notifications, when a bounce message size exceeds
the local or remote MTA's message size limit.
It documents the default, but not the maximum. Apparently there is one (and
who would care, one of Debian's users, apparently [1]). I'm not particularly
confused about why there would be a maximum, but it might be reasonable to
document what it is. Perhaps add something like "Maximum value is
2147483647." at the end of the note so that users don't have to find out the
hard way:
fatal: bad numerical configuration: message_size_limit = 2147483648
Scott K
[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=960272
