Search on the term "autogrow" in Books Online, and you should find this in an article titled "Physical Database Files and Filegroups":
Originally Posted by Books Online
SQL Server 2000 files can grow automatically from their originally specified size. When you define a file, you can specify a growth increment. Each time the file fills, it increases its size by the growth increment. If there are multiple files in a filegroup, they do not autogrow until all the files are full. Growth then occurs using a round-robin algorithm.