So what should you use for a stripe size? The best way to find out is to try different values: empirical evidence is the best for this particular problem. Also, as with most "performance optimizing endeavors", don't overestimate the difference in performance between different stripe sizes; it can be significant, particularly if contrasting values from opposite ends of the spectrum like 4 kiB and 256 kiB, but the difference often isn't all that large between similar values. And if you must have a rule of thumb, I'd say this: transactional environments where you have large numbers of small reads and writes are probably better off with larger stripe sizes (but only to a point); applications where smaller numbers of larger files need to be read quickly will likely prefer smaller stripes. Obviously, if you need to balance these requirements, choose something in the middle. :^)
Note: The improvement in positioning performance that results from increasing stripe size to allow multiple parallel accesses to different disks in the array depends entirely on the controller's smarts (as do a lot of other things in RAID). For example, some controllers are designed to not do any writes to a striped array until they have enough data to fill an entire stripe across all the disks in the array. Clearly, this controller will not improve positioning performance as much as one that doesn't have this limitation. Also, striping with parity often requires extra reads and writes to maintain the integrity of the parity information, as described here.
