For Qmail, the default queue location is /var/qmail/queue, not /var/mail or /mail; if you're using the qmailrocks instructions, mail will reside in /home/vpopmail/domains/domain/user/Maildir, or /home/localuser/Maildir. For this reason I'd make /var and /home separate partitions... perhaps 10GB for /var, and whatever's left for /home. You can probably get away with less in /var if you don't keep as many logs around as we do... but if qmail runs out of room for the queue, I imagine things would get ugly.
If a dedicated mailserver running qmail is swapping, something has gone terribly, horribly, awfully wrong.
We usually leave off a swap partition... but that's when using 9 or 18 gig SCSI drives; you've certainly got the room to spare.