First off use 7.3 as it has changed since 7.2 - see the change log for details on this site.
I was in a similar position and asked our account manager about this and it's more to do with the amount of requests/ second that the appliance has to deal with than the amount of users although there is a relationship between the two. There are also other factors to consider:
• Number of AV engines - one or two?
• Is the SSL Scanner used? If so, what’s the percentage of HTTPS traffic?
• Is Web Cache used?
• What type of authentication is required?
• How complex are the policies?
• Maximum capacity required?
I compared the stats and reviewed the performance of the existing 6.9.1 build against the hardware appliances to come up with a suitable config. You also need to consider what your strategy for log management is as if kept locally, then you will eat up your disk space over time.
I found that with the existing setup (on a hardware appliance with 2gb RAM/ Intel Core 2 Duo CPU @ 2.20GHz) running about 130 rps was getting maxed out so I specced out a VM with 8gb RAM and a Xeon quadcore which should be sufficent. I based this on:
• 10% HTTPS traffic but not currently scanning HTTPS traffic - this will add overhead.
• Full McAfee AV coverage with Proactive Scanning.
• Web Cache.
Aim to be @ maximum of 80% capacity
My VM architect only gave me 4gb RAM however as he said that as long as your VM host has enough processors and memory you can always increase the RAM/ CPU if the initial config isn't sufficient. I think I will need to increase the RAM once I move out of UAT into production.
Additionally, I guess you have seen page 20 of the Product Guide?
Requirements for setting up a virtual appliance
To set up a virtual appliance the following is required:
• One of the following VMware types:
• VMware ESX
• VMware ESXi
VMware workstation version 5.5 or later is not supported, but can be used for testing purposes.
• Virtual machine host system with the following specifications:
• CPU: 64-bit capable
• Virtualization extension: VT-x/AMD-V
• Virtual machine with the following specifications:
• Memory: 4 GB
• Hard-disk space: 200 GB
• CPU cores: 2 (minimum)
Hope this helps