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Oracle Minimum Block Size

Minimum block size should not be less than 8K.
Oracle performance benchmarks show that every type of Oracle application will benefit from larger block sizes, even if the rows that are being requested are very small. This is because Oracle index searches retrieve adjacent rows on a block, and because Oracle index range scans often request multiple rows on a single block.
The Oracle Database Administrator's Reference 10g Release 2 (10.2) for UNIX-Based Operating Systems notes these guidelines for choosing blocksizes in AIX:

Smaller Database block sizes for OLTP

Oracle recommends smaller Oracle Database block sizes (2 KB or 4 KB) for online transaction processing (OLTP) or mixed workload environments and larger block sizes (8 KB, 16 KB, or 32 KB) for decision support system (DSS) workload environments.

Adjusting Oracle Block Size

During read operations, entire operating system blocks are read from the disk. If the database block size is smaller than the operating system file system block size, then Input-Output bandwidth is inefficient. If you set Oracle Database block size to be a multiple of the file system block size, then you can increase performance by up to 5 percent.
The DB_BLOCK_SIZE initialization parameter sets the database block size. However, to change the value of this parameter, you must re-create the database. To see the current value of the DB_BLOCK_SIZE parameter, run the SHOW PARAMETER DB_BLOCK_SIZE command in SQL*Plus.