No of tables in a database doesn't specify the size of the database.Because, One can have one table that occupies 2GB or 200 tables that occupie 2GB.So, Its the space(in MB/GB/Tera Bytes) occupied by all datafiles on the OS file systems.
There are other factors like No of concurrent users, transaction volume adds to scalability and capacity of database.
If you have'nt yet created your database, you need to do
a proper table sizing exercise, to guesstimate the amount of disk space you will need.
Some Things to consider Per Table :
1. No of bytes per row
2. Number of rows initially, and add the forecasted growth of the table - saves you having to add datafiles all the time, and
3. Number and size of indexes