On a RAID controller, if you do a read/write request that's bigger than a single stripe, it has to be internally split into multiple reads/writes by the controller. So, for systems doing many small writes, you want as many writes as possible to fit into a single stripe (so that each write can be serviced by a single disk), but if the stripes are too large, you'll have problems with multiple reads/writes going to the same stripes simultaneously (which bogs things down considerably). So if you know (for example) that you're running a database server that reads and writes 4K pieces of data to disk all day long, you'd probably want a 4K or 8K stripe size.
For systems doing many large reads/writes (a video editing workstation, for instance), you want the stripes as large as possible, so there is less overhead in the RAID controller (the large reads/writes will get split fewer times with a larger stripe size).
Unless you're tuning the array for a specific application, just leave it at the default (probably 16KB or so). Feel free to play around with it, but I doubt you'll see much improvement for 'general' usage.
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