At this point, you may be using your imagination and having daydreams about entertaining on your new extended deck. As much as we hate to burst your bubble, we have to recommend that you check on your local building codes and permit requirements. Do this early in the process so that you know what limitations it may pose. The codes from your local building department or homeowners association might limit the size or location of your expanded deck (perhaps that's the reason why it's so small to begin with!). You'll also need to contact your utility companies and ask them to send someone out to your house to mark any underground utility lines.
Before widening or lengthening your deck, it's vital to ensure that the original deck is structurally sound: Check the framing for any signs of rotting, cracking or other degradation. If the deck is attached to the house (not freestanding), make sure the ledger board is properly secured to the side of the house. Also, inspect the posts underneath that support the deck for evidence of having heaved and thawed. Even if you're simply adding a new level to the deck, and the existing foundation won't be used to support the new portion, use this renovation as an opportunity to update the old deck if needed.
When planning your deck layout, use mason string attached to batter boards or wood stakes. Contractors will use this method to keep the deck level and square, but you can use it in the initial planning stage to visualize the placement of the deck. Consider space issues, and set up your desired lawn furniture inside the mason string area. Don't forget about sun exposure and how much sun or shade your deck extension will get.
Even if you plan to hire a contractor to install the deck extension for you, it's a good idea to do all these steps anyway, so that you know what to expect and can be as precise as possible when negotiating and explaining what you want.