Microsoft has provided a nice easy command to show you how large your current databases are,  this one line command can be pipped to a file or used in a report

Get-MailboxDatabase -Status | fl name,databasesizes

Name         : DB001

DatabaseSize : 141.8 GB (152,220,270,592 bytes)

 Name         : DB002

DatabaseSize : 98.17 GB (105,412,362,240 bytes)