I'll second 16k/16k
Its a good starting point, and will usually give good overall perfomance untweaked. BUT, you can always spend the time and squeeze that last ouch of juice out.
In general, smaller file arrays (OS, normal files, normal use) will benifiet from smaller stripe/cluser (16/16 and under) Where larger multimedia files could see a increase in larger pairings. Overall, 16K is a safe place