The first place to look at is your database configuration. Your database must be configured with a default database block size. The database block size is the smallest unit of I/O. The db block size can be 2K, 4K, 8K, 16K (and 32K on some platforms). Notice that 6K is not present in the list. Therefore, if you are considering 6K or 8K, I would strike 6K from the list automatically. When reading an entire table, the database will read multiple blocks at the same time. The number of blocks requested at the same time is defined by the DB_FILE_MULTIBLOCK_READ_COUNT parameter. This parameter is normally set to eight and should only be changed if you have a very good reason to do so.
If you have not created the database yet, then many people are going with an 8K block size for their databases.
This was first published in December 2006