Did you know the default autogrow (filegrowth) settings for SQL Server is 1MB at a shot? That can really hurt your SharePoint Server performance. The best practice is to set the inital size to your max size, like 100GB. But, if you must use autogrow, here is a sample script you can run in SQL Server Management Studio. Note that it also sets the initial size. I would make that initial size something like 50GB.
Use Master;
Go
alter database Contoso_WSS_Content_Portal_01
modify file
(NAME = Contoso_WSS_Content_Portal_01,
SIZE = 1024MB,
MAXSIZE = 51200MB,
FILEGROWTH = 1024MB);
go
That will change the data file 'Contoso_WSS_Content_Portal_01' to 1GB initial size, with 1GB autogrow increments.
Thanks to Brian Alderman for the help tuning the above. Also, check out Mike Watson's blog here. He and Bill Baer really know their SQL!
Ben