Hi @vhsieh ,
It is difficult at this point to come up with a number as it varies from client to client, environment-to-environment. It will depend on a lot of things like how big or small the TM1 servers are, and user activities, etc.
We do have some client before who started with SQL Express (maxed at 10GB). For this one, you could recommend to start with a size (i.e. 10GB) that they are comfortable with, and review it after a month to upgrade if necessary.
You can also then combine it with the Maintenance Job of Pulse.
Again, these are just guidelines/estimates and may need to be adjusted in time.
Cheers!
Paul