Deck for every activity
Like many other questions about building a deck, the answer to how large it should be will pretty much fall into place once you've decided how you're going to use the deck.
If your deck will have several uses (a play area for the kids, space for dining, an area for sunning) you may be tempted to make it as big as possible. But multiple functions don't always call for more deck. Sometimes a small design element, such as a level change, can better isolate functional zones. However, if you plan to have a one-level deck, use the high end of your size estimate. Adding more space now is less expensive than adding it later, and the last thing you want is a cramped deck. If your budget is tight, plan a deck that you can build in stages, adding elements as you can afford them, but keeping the overall design intact.
The chart on the right shows rough estimates of the space required for different activities. Use these estimates as a guide to sizing your deck.