The below is from the VI install guide. How large is the data file and how much free space do you have left in it?
Storage requirements can be larger if your database runs on the same hardware as
the VirtualCenter Server machine. The size of the database varies with the number
of hosts and virtual machines you manage. Using default settings for a year with
25 hosts and 8 to 16 virtual machines each, the total database size can consume up
to 2.2GB (SQL) or 1.0GB (Oracle).