they are usually 6 inches long but the smaller they are the harder it is to throw them from a far distance.
http://www.secrets-of-shuriken.com.au/index1.htm
i have read from a couple of sites that 8" is one of the best lengths but dont go over 14".
http://www.throwzini.com/joe_a_spikes.html
i think that bo shurikens are better than shuriken (stars) or throwing knives
you can throw them just as far as a knive or star (still hitting the target) easier to hide without stabbing/cutting yourself, and they just look sweet in the air
but thats my opinion