You are seeking some guidance on allocating disk space for Symantec Messaging Gateway (SMG) Appliance Virtual Edition, formerly known as "Symantec Brightmail Gateway (SBG) Appliance Virtual Edition."

Disk space requirements vary widely and depend on various factors including but not limited to log data, reporting data, mail volume, Content Folders, Spam Quarantine, Message Audit Logs, etc., and the period of retention.

For most installations, the minimum recommended disk space of 120GB should be sufficient, but the above factors should be considered for the environment. 

Another approach in allocating disk space is to mimic the disk size available on the SMG hardware appliances.  For details on the SMG hardware models, please see the product Data Sheets available on the Symantec web site.

Symantec Messaging Gateway 10.8.1 Installation Guide

If you find that your SMG needs more disk space, there is a way to expand it, but only under certain circumstances and only in virtual environments.

How to expand the Messaging Gateway disk on virtual infrastructure