A majority of all controllers on the market when they detect the hard drives you are using to setup a raid array.  Leave it at default, it will detect at either 64k or 128k file blocks.  You will only ever want to change this under very hyper specific circumstances.  If you change it and your application does not coincide with the higher or lower file block size you will have performance degredation, possibly severe degredation.

What is File Block Size?  That is the size of data blocks the array is segmented into.  So if you have a 1mb file and a 128k file block size the file will be segmented into 8 pieces.  If the size is too large you will have alot of wasted space as well as HDD latency will go up.  If it is too small, there will be alot more processor overhead and the system as a whole will slow down.

Your best off letting the raid controller pick what is best for your specific array.