one of the challenges for many organizations is that things change.
1.How many mailboxes will reside in the database? Changes over time
2.What is the message profile of the mailboxes in the database? Changes over time
3.What is the average message size? Changes over time (usually not too much)
4.What is the average mailbox size? Changes over time
5.How many mailboxes are moved per day? Depends.
6.What is the backup and restore solution? Usually static.
7.Does the solution need to take into account any other failure scenarios, like network failures? Always!
So you capacity plan today and how often do you update that plan...when you run out of trans log space and get bit by it.
A litigation hold or Single Item Recovery significantly changes this growth too...