I didn't know that was important while building a database schema?
We are talking about a relational database design, right?
Sizing usually comes afterwards.
Anyway: There are some default models available.
I 've seen one on the RDD course of Informix Software.
I was thinking of which technologies first although the way you might want to use the database and the amount of data might effect how you design the structure. (i.e. you could contain multiple sets of 'lookup' data into one table, or have one table for each set of 'lookup' data')
or you might want to use partitioned tables (oracle data warehousing)
I've not worked on call centre systems - sorry for not being much help