1) how much space should i take into consideration is needed for the / partition?

if you don't plan to install many applications on this server, i guess 20GB is OK.

The directory used to store SQL database?

Depends on how many emails it will process per-day, At least 2GB is required for production server. But, the more the better.

Im about to install iRedMail-Pro this weekend, and want to make the correct partitions from the start.

I was planning to make a seperate partition for /var/vmail, /var/log, /swap and /boot (which makes it so muche easier to increase space later on in my VM ware setup.)

Question is:
1) how much space should i take into consideration is needed for the / partition?
2) Are there any other partitions i should consider?
3) How much ram is it recommended i allocate to the machine?
4) As i understand quarantined mails are stored in MySQL - so i guess a /var/lib/mysql partition might also be an option, so it can be resized if needed? (according to our current quarantine stats around 65% of mails recieved are clean)

Bonus info: The server will be handeling around 400k mails / month, spread over 400 domains, and around 650 mailboxes.

i hope someone can give me a quick point in the right direction....

Thanks in advance from a newbiee when it comes to iRedMail