The size of the data block can be set at the time of the creation of the database. The default size of 8 KB is adequate for most databases. If your database supports a data warehouse application that has large tables and indexes, then a larger block size may be beneficial.
If your database supports a transactional application where reads and writes are random, then specifying a smaller block size may be beneficial. The maximum block size depends on your OS. The minimum Oracle block size is 2 KB and should rarely (if ever) be used.