When it comes to the size of your deck, think about the size of your garden. How much of your space do you want to cover with decking? For example, if you have a large lawn area do you want just a small decked area off to the side, or do you want a larger decked area taking over the grass?
Perhaps you love the idea of decking as a low-maintenance alternative to a lawn? If in doubt, go smaller, you could always add decking on after.
You might even consider going ultra low maintenance and installing composite decking and artificial grass! This way you have much more flexibility and less maintenance.