AFAIK, its nothing to do with how big is the database. Its depends on what kinda of applications, howmany concurrent applications, its in distributed environment or not, 24X7X365 avaialbility, no of users etc will create the necessity for use of n-instance 1-vldb OR RAC (OPS).

There are 1-instance and 1-vldb configurations exisits with top of the level hardware configurations using Sun12K or IBM S80 and other top of the level machines around the world...