Compact and directive ultra-wideband antennas are required in variety of applications. Directional wideband antennas can be designed by using a reflector to redirect the energy back in half space and increase the gain. Use of artificial magnetic conductors (AMC) as reflectors for antennas allows reduction in the thickness of an antenna using traditional perfect electrical conductors (PEC) reflectors. The lateral size of the reflector also has an important effect on the antenna performance. In this paper, we study the trade-offs involved in the design of an AMC used as a reflector for broadband diamond dipole antenna by simulating various sizes of the reflector.