In a case like that I would look to the supplier/manufacturer of the steel deck for guidance. I would not base my design on an aspect ratio limit, particularly since these decking profiles can be so different from one another. Contact VicWest directly (or whoever is the manufacturer of the particular steel deck). As a rule of thumb, I always check the deflection of the span I'm proposing against a 40psf UDLoading. Seems arbitrary, and it is, but it's good enough for a rough preliminary sizing, which is what I'm guessing you're after by asking for an aspect ratio...
As an aside: Remember that limiting your aspect ratio is almost never realistic... You really need to ensure your total system (steel deck and the supporting structure - ie: beams, girders & columns) are suitable against serviceability. In the case of a floor system, and probably quite adaptable for a roof situation, your best best is to look at the CISC/AISC Design Guide 11 - Floor Vibrations Due to Human Activity. It's available for purchase from AISC's online store. Pub. No. D811 (10M797).
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
