This is the size if bytes for 'chunks' and is only relevant to raid levels that involve striping (1,4,5,6,10). The address space of the array is conceptually divided into chunks and consecutive chunks are striped onto neighbouring devices. The size should be at least PAGE_SIZE (4k) and should be a power of 2.
This can only be set while assembling an array.
See also: RAID_setup#Chunk_sizes