A good question to ask when designing your deck: what indoor room am I copying? If you plan on a lot of barbecuing and eating your kitchen is the answer. If relaxing in the sun is more to your liking, the living room is a good choice.
Take a measurement of the room that applies and you've got a good rough estimate of a deck size that will be good for your needs.
Another good rule of thumb for deck size: 20 square feet per person plus one person more. For a family of four this would come to 100 square feet (20 X 4 + 1).
Traffic flow is another design consideration that shouldn't be overlooked. Make sure the steps and exits are naturally accessible with a minimum of disruption to people relaxing and enjoying the deck.
Spare a thought for the elements. If your backyard is particularly sunny you might want to add a pergola to block the heat. Or if there isn't a lot of privacy in your backyard and you like to tan you can build privacy screens to make it a go.
Form always follows function. Think of how the deck will be used and plan accordingly. For instance, if you have a lot of friends and plan on a lot of dinner parties you'll want a lot more space than you'd normally consider for just your family.
A final thought: the focal point. Most decks are built off to the side instead of dead center of the back of the home. The reason: it makes your yard look bigger. Both from the deck, and from the yard. It's one of those old tricks that is so popular people just do it without realizing why.
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