I had a call today, a buyer was asking about spans for joists on his decking, using 2×8 and/or 2×10, how far could he go? I don’t think that is the right way to approach the problem, there are span tables recognizing various species have different strengths , and then there are recommended spans for various weight loads. The real questions are what are you putting on the deck in terms of weight, what species are you using, and what margin of safety do you want. Take a look at your options.
A good resource would be the West Coast Lumber Inspection Bureau that has a simplified table for floor and ceiling joists with giving how much a distance you can use joists, at recommended loads. – It is also important to know what species you will be using, different species have different strengths.
Also, check with your local building code and inspectors first before doing anything. I believe in the theory of overbuilding, if you think you will have a large number of people on your deck for BBQ’s or parties then take that into consideration. I don’t want anyone finding out after the fact they have a problem.