How much weight should a deck of a given area support?
I found the following VERY detailed tutorial on determining joist span based on expected live and dead load: American Wood Council

The weight your deck will need to support will depend whether you get snow and how much, whether there will be a roof over the deck, whether the deck will be sheltered by any other means, what do you expect to do with the deck (outdoor kitchen, hot tub, outdoor parties), etc.