John, are you attaching the joists directly to the posts or are you planning to bolt beams to the posts then set the joist frame over the beams? The latter is better because your joists won't have to span as far plus, you can cantilever the joist frame over the the beams about 1.5' without upsizing the joists. It makes it look like it's floating in air a bit.
Most big box home improvement stores have simple CAD programs that they can design your deck/dock for free. They'll usually print you some decent working drawings (usually a 3D too) and a material list. The parameters of these programs can be easily changed to accomodate different sizes of lumber.
