guidelines for db_block_size

Verdict
~~~~~
Use db_block_size = 2048
only if you really know what you are doing. The best use of 2KB database blocks that I know is in stress tests in which you are trying to drive server workload artificially high so you can analyze the bottlenecks less expensively (i.e., without generating mountains of test data and test transactions).

Use db_block_size = 8192
for most large transactional processing systems. This represents a good balance between advantages and the disadvantage for undo segments.

Use db_block_size of larger than 8KB
for OLTP systems in which your data structures drive the block size to a naturally larger size. This will help to to avoid chained and migrated rows.

Use db_block_size of larger values than 8KB
for systems in which your undo generation is not a meaningful part of your workload. Data warehouses fit this profile. With bigger blocks, you reduce total system I/O setup costs dramatically, yet you incur none of the disadvantages that you would incur in an OLTP system because people generally are not executing transactions (inserts, updates, deletes, and selects for update).

The maximum size of a single index entry is approximately one-half the data block size(8i only).
block size maximum size of a single index entry
(tested on RHEL2.1, oracle8.1.7)
~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4KB 1578bytes
8KB 3218bytes
16KB 6498bytes

Data block size affacts the size of SGA(8i only)
The size of the SGA is determined by several initialization parameters. The parameters that most affect SGA size are:
DB_BLOCK_SIZE: The size, in bytes, of a single data block and database buffer.

DB_BLOCK_BUFFERS: The number of database buffers, each the size of DB_BLOCK_SIZE, allocated for the SGA. The total amount of space allocated for the database buffer cache in the SGA is DB_BLOCK_SIZE times DB_BLOCK_BUFFERS.

Please make a full and up-to-date backup of your database before export and import