Make the spacing (the actual gap between) the same as a baby crib. That is, a standard soda can (Pepsi or Coke) should not slip through the open gap.
But Jeff, why not make a call to your local building inspector and ask him? Make the call anonymous if your worried about it. They are usually glad to help. One other thing, the space between may even require a lip to prevent objects from rolling off the deck and hitting someone on the head walking below