Hi, found the following sizing guide posted by Anup Patwardhan. It appears to be quite detailed but was posted back in 2009. Is that still current with the latest versions?
6 Tips for a Smooth Zimbra Server Install :: Zimbra :: Blog

I have 8,777 users and if I plan for a 10,000 users installation with 1GB allocation per user. Here's what I worked out to be. Is that correct? Traffic will be quite low as not every one will be actively using mail. Performance doesn't have to be top notch. Is it simply a matter of adding more disks or do I have to be concerned with not being able to run all of the users on one server, say a Haswell Core i7 with 16GB RAM?

Item Number Sub-Total (GB)
Quota / User (GB) 1
Users 10,000 10,000
MySQL 5% 500
Binaries 10
Logs 20
Indexes 25% 2,500
GB 13,030
TB 12.75