What is the optimal buffering size?

bsopt.sh

#!/bin/bash

echo "creating file to work with"
dd if=/dev/zero of=/var/tmp/bsopt count=1145140

for buff in 1M 2M 4M 8M 16M 32M 64M 128M 256M

do
	echo "---------------------------------------"
	echo "Testing block size  = $buff"
	dd if=/var/tmp/bsopt of=/var/tmp/obsopt bs=$buff
	echo

done

echo "success :)"

echo $t

rm /var/tmp/bsopt /var/tmp/obsopt