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.
Questions? Thoughts? You can reach me at firstname.lastname@example.org. I'm here until Christmas...