From within vCenter Server, on the menu bar, select Administration -> vCenter Server Settings-> Statistics there is a section for Database Size. Change values such as collection intervals, # of hosts and number of guests (virtual machines). Change these values to match your environment and it will display the estimated database size.

Click the Help button for more help on Configuring Statistics Settings.

VMware has a Database Sizing Calculator tool on its website. Current version works for vSphere 4. http://www.vmware.com/support/vsphere4/doc/vsp_4x_db_calculator.xls

To manage the growth of the vCenter Server database, you may choose to configure the database to discard tasks and/or events information after a specified period of time.