suggested size of /var/mail

[D G Teed]

No one qualified the context of the question.  Not the OP, nor the answers.
Amateurs are you all?

Details, details, details!!!!!

Is this for home, or for a hobby site, or the real world?  What is the
volume of use?  What sort of server is it - MX, apache,
database server, America's Army run in Linux compatibility mode, etc.?

No one can answer partitioning without knowing what you are doing, and where.

If it is a personal server, with very few or only one user on it, then
you might not even need a separate partition for /var.

If it is a production server of some sort, strongly related to that
gray cloud Internet thing, then having a different
partition for /var which can fill up with space or inodes
and you can still login to the system is the best way to go.

I've seen /var fill up a few times on various servers and this is
a good measure for a vat that can take the excess and then stop.
Anyone who says they never seen a problem with this has not
run something like a web server for more than 3 years in the
real world (by this, I mean a domain name that people around the
world know of).

As for how big, no one can guess.  You have to think about it.
If you've never done this install before, perhaps you'll need to
do it twice to get the partitioning right.

It certainly isn't like Solaris, where the copies of every patch you've
ever applied are archived for eternity, consuming up to 6GB or more.

--Donald