A block size of 8 KB is optimal for most systems. On the other hand, OLTP systems can use smaller block sizes (such as 4K) and DSS systems can use larger block sizes (such as 16K, 32K).
Guidelines for database block size:
1) You need to set Oracle Block size equal to or a multiple of the operating system block size.
2) If your rows are small and you use a large block size, when you fetch a block you may get lots of rows and waste your buffer cache. It may also cause block contention.
3) If your rows are large then you may need a large block size to prevent “chaining”.
4) Larger Oracle block sizes may give better index balancing with more rows in each branch.