Need advice for partitioning, sizing HD

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- iRedMail version:
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):
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I am about to install CentOS 6.4 (64bit) on a 4TB drive.
I will be using the server with IRedMail (MySQL).
In order to have the max user space (mailboxes) and max number of users, how do you suggest I partition the HD? And I allocate space for the mount points?

Thanks in advance.

Omer