Decide what you hope to use the gazebo for when deciding on the size of the gazebo design. If you plan to use it for outdoor entertaining of many people on a fairly regular basis you will want to be sure it accommodates as many as you can. Additionally, you may want to consider multiple types of seating to accommodate a large number of people. This can be tricky because gazebos are typically round in shape. You may want to consider putting additional seating around the garden area or the outside of the gazebo. If you plan to use the gazebo for special occasions consider what will be needed.
When you are working on your gazebo design be sure to keep in mind the overall design of the house. You do not want to break from that style when working on the gazebo. You want it to compliment the house not take away from it. If your home is classic you may want to keep the gazebo old fashioned and traditional. The same applies if your home is more modern or perhaps is of a Spanish style, you will want the gazebo to reflect that same style. Planting is fairly important when working on the gazebo. Be sure you know what plants will surround the gazebo. This will help you to plan the size and shape of the gazebo better.
While you certainly can draft a design yourself, you can also find a number of terrific options on the internet to choose from. Even if you are just looking for some great ideas, there is a wealth on information online. Finding just the right gazebo for your home is important because it is the one thing in the yard that could make your neighbors and passersby say "wow look at that". Think hard about what you would envision in your yard for a gazebo next to your house and there you will have your gazebo design.