Docs say
[i]
set DB_BLOCK_SIZE multiple to OS block size but it should not exceed the maximum size of OS I/O buffer. 
[/i]
why?

db_block_size helps with database I/O. 
Operating system block size helps with OS I/O
so if OS Block size = 8k
and Database block  = 4k
for every 2 database calls, One OS block is used.
if osblock=8 and dbblock= 16
then
	for every database call, 2 osblocks are used.

and db_block_size should be based on nature of the database.

for OLTP system use smaller blocks
for DSS /warehouse use larger blocks.

>>Hoow to know os block size?
depends on your Operating system.
in solaris i would use
bash-2.03$ df -g | grep "block size"
/                  (/dev/dsk/c0t0d0s0 ):         8192 block size          1024 frag size
/proc              (/proc             ):          512 block size           512 frag size
/dev/fd            (fd                ):         1024 block size          1024 frag size
/etc/mnttab        (mnttab            ):          512 block size           512 frag size
/var/run           (swap              ):         8192 block size          8192 frag size
/tmp               (swap              ):         8192 block size          8192 frag size