Size reduction for a single antenna element is desirable for wireless personal communications. Here, a 750/0 size reduction of the dielectric-resonator antenna (DRA) is achieved while maintaining the antenna's bandwidth at the same time by incorporating the concepts of an electric conductor and a magnetic conductor. To validate this concept, infinite perfect electric conductor (PEC) and infinite perfect magnetic conductor (PMC) planes with the presence of the source are used in an ideal situation. A finite electric conductor and a high-dielectric-constant interface are then applied to reduce the dielectric-resonator antenna's size in a realistic situation. The antenna's size is reduced while keeping its frequency band within the original band of the full-size case. For wideband elements, the feed probe in a low-dielectric interface is used to support the concept of magnetic walls. The concept was first confirmed by the examples of full-size and reduce-sized cylindrical dielectric-resonator antennas. It was then applied to rectangular-cup dielectric-resonator antennas, simulated numerically, and confirmed experimentally. More than a 30% impedance bandwidth was achieved by a quarter-size case with a 50 ohm hook-shaped coaxial probe placed off the antenna's axis. Results indicated that a 750/0 size reduction could easily be achieved for the dielectric-resonator antenna.