All dogs require exercise to lead balanced, healthy lives.
Just because a dog is small doesn't mean he needs less exercise, and, although breed can give an indication of the dog's energy level, the final judgment must be made on the individual dog's needs.
As your dog's pack leader, you must help her to expend her energy in a productive way. For all dogs, this means a daily walk. Some dogs may require additional activities, such as running, Frisbee, or swimming, in addition to, but never replacing, the walk.
Remember that dogs are animals, and animals need to travel and discover the world. A big back yard is no substitute for a good walk. Master the walk and your dog will relate to you on a deeper level.