Remember you will need space not only for plants but for the rocks. The rocks are important to create crevices and fast draining areas. Rocks will also provide shade and cool places for plants that require these types of areas. Rocks are the main items in the rock garden besides the plants. Make your rock garden as large as possible, especially if you do not have a sloping area.
Soil type is probably one of the most important parts of constructing a rock garden.
Well drained soil with the correct pH for the plants you have chosen will determine your success. Quick drainage is important but so is moisture retention. The plants will benefit from occasional additions of manure or compost to the soil. For a good soil mix we recommend reading H. Lincoln Fosters book on Rock Gardening. These are the most important factors, but there are one or two more things worth talking about.