Just stay away from the "bargain" cables,
a decent cable should be around $20.00 at least, although they sometimes go on sale for $15.00.
the length souldn't matter on a mic cable, they are balanced and low Z which means you can run them for great disances without signal loss (example: the "snakes" used onstage at large concerts are 100-150 feet, plus the mic cable onstage)
Of course some recomend Monster cables and the like, but don't spend that kind of money until you've learned how to hear the difference.