Mark off the deck area using string. The string will help you visualize the size and appearance of the finished deck and will also serve as a guide for excavation and post placement. But trust me, it will always look smaller this way than when it is done.
You can build this sturdy bench in an afternoon. It makes a good edge bench to define the ends of a low deck, or a bench that you can use near a pool is edge or even against a railing. Its clean simple lines would make it fit in with most any decor.
At the link you will find lots of free information to get you on your way to building a deck of your dreams.