Many first time deck designers make the mistake of creating deck designs that are too large to fit the scale of their house and exceed their budget. Most professional deck builders use a few rules to help scale the deck to accurately fit the house.
1) A deck shouldn't be built larger than 20% the square footage of the house it is attached to.
2) No single section of the deck should be larger than the largest room in the house.
Using these rules, get yourself a notebook of ¼" graph paper and a ruler and start sketching out your ideas. Consider designing a multilevel deck with a few separate areas to break up a large boring rectangle. Try to let deck areas flow together in a natural and simple way.
Sometimes it is difficult to look at a deck plan on paper and understand exactly how it will look and feel in real space. You may need to layout the design at the location to be able to get comfortable with its shape and proportion. Use a tape measurer and a garden hose or some spray paint to layout the design's outline or footprint. Does it feel larger or smaller than you anticipated? You may want to arrange some furniture within the outline to help you visualize how the space will be used. Will you have enough space to move around the deck freely or will it be too cramped? You may need to make some changes to the plan in order to adjust to your findings.