Deck Stair Code Information from IRC 2009
R311.7 Stairways: The minimum width for stairs is 36". The maximum riser height is 7-3/4" and the minimum riser is 4". The minimum stair tread depth measured from nosing (also the stringer rough cut) is no less than 10". The tread width and riser height shall not exceed the smallest by more than 3/8". This means you will have to plan ahead to make sure every one of your stairs measurements are the same.
R311.7.7 Handrails: Stairs with 4 or more risers must have a handrail located above all portions of the treads, between 34" and 38" above the tread nosing, continuous between floors, with ends terminating at wall or newel post so as not to snag clothing. Circular handrails must be between 1.25" and 2" in diameter. Other shapes with a perimeter dimension between 4" and 6.25" with no cross sectional dimension greater than 2 1/4" are acceptable. Rails with a perimeter greater than 6.25" must have a finger grip routed in both sides. Handrails must extend from the top to bottom riser. Note: A 2x4 does not meet handrail cross section requirements.
R312 Guards on Stairs: Open sides of stairs with over 30" rise above the adjacent floor or ground must have a guardrail at least 34" high measured vertically from a line connecting the leading edge of the stair treads. Required guards shall have balusters or other members arranged so that a 4 3/8" sphere cannot pass through them, except that a triangular opening formed by the guard, tread, and riser can be such that a 6" sphere cannot pass through it. Open risers are permitted by code with a catch because they may not be large enough to permit the passage of a 4" sphere.
R312 Guards on Decks: Decks with over 30" rise above the adjacent floor or ground must have a guardrail at least 36" high measured vertically above the deck or fixed seating. Openings shall not allow passage of a sphere 4" in diameter.
Note: Stairs cannot be more than 12' high without landings, (19 treads) R311.7.5