A Deck like this is simple, design for 50 lb per sq ft load. That is the same as most floors. The real concern are couple other things the support and the water proofing to the home.
The support foundation should have enough depth into the ground. Me? I would make them out of 12" round 18-24 " into the grade. Make sure the post to beam have correct metal brackets to support. The framing needs to be independent enough so it is not dependent on the house, In a hurricane or earthquake the asymmetrical reaction force will cause them to move differently, if the deck is dependent on the building as a mean of support, it fails. Saw this alot in the Loma Preita earthquake. Look up floor joist spans for 12' @ 24"centers, my guess it would be 2x10. One other thing, if you use a composite decking, I would go 16", this prevents any kind of sagging due to extreme heat and weather.
The other thing is the connection to the home, what has been the standard the last 10 years is using Ice and water shield (bithuthene) and a z- type flashing. Then, they usually go with a 2-3"wall side up with a 1/4-1/2"90 bend kerfe'd into the building set in a Low modulus (Sika 15lm or equal) caulking.
See the attached photo. I have done this detail at least 2000 times. Seems like alot, but this been the standard in the west coast for a long time. Mostly done on Apts or Condos, .