A good source for building resource information by: Editor - Rich Bergman
Sorry for the delay in responding. I think one of the first places you might go to is the American Wood Council at www.awc.org.
They have a ton of great resource material which has span tables done up by experts that are referred to in building codes. I do think that is the most efficient direction I can give you.
They also have a great new publication on deck building called DCA 6 which sells for about $10 and has super comments in it including joist and beams spans and spacing.
I refer to many of their publications often as they are done right.
I won an inflatable 6 person inter hot tub. I have a large deck. 12x 20 on both sides of the house. With a header to the house and the deck was built 6
I'm putting a swim spa into an inground concrete bunker and will build a deck around it. I have been advised to keep the deck joists a minimum of 6 inches