Ext3 Partition Optimized for RAID5

Check the output of mdadm. It tells you the block size in chunks, which you will need.
mke2fs -j -E stride=64,stripe-width=320 -L LABEL /dev/md0

In my example the chunks size is 64k. I use 6 disks, out of which 5 carry data, so the stripe-width is chunks*5=320. See man mke2fs for more.

I always add this for even more speed:
tune2fs -o journal_data_writeback -O dir_index /dev/md0