I own a landscaping business and we also build decks, piers, arbors, etc. As mentioned in the above post you can set your posts on 8' centers using 2x8's. I would also recommend using 5/4 deck board instead of 2x6's as your floor boards. The reason for it is wood when wet from rain acts as a sponge and it takes longer for 2x6's to dry since they are approx. 1.5" thick compared to 5/4 deck boards that are 1" thick. Also, the 5/4 boards have a nice rounded edge. The biggest thing to remember is the new treated wood today contains copper so you "have" to use "hot dipped galvanized" or "stainless steel" hardware instead of the regular zinc plated nails and bolts. Also, make sure you use bolts where the outside lower joists make contact with the posts. Nails are not enough.

Email me if you need any advice. Good luck!