This is a very open question. No one can just give you a approximate size for a few factors are involved here such as;
Data being put in the db
Total Number of tables
Total Number of temp tables
Data that is being retained
So there is suppose to be database sizing tool/excel sheet from the application owner, for instance if you compare it to 5000 machines running Symantec Antivirus, these machines will be consuming 40Gigs atleast in an year. So you see, this all boils down to how much data is being put in the db.
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