In this study, a single layer reflectarray antenna is proposed with variable patch sized unit cells and suggested for enhancing the bandwidth with small overall dimensions for X-band applications. The novel unit cell consists of double cutted ring geometry. 13 x 13 unit cells reflectarray is covering an 162.5 x 162.5 mm(2) ultrathin square substrate with epsilon(r) = 6.15 dielectric permittivity and 0.042 x lambda(0) thickness. In order to obtain larger and smoother phase curve, 2 mm air gap is inserted between substrate and ground plane. The required phase delay is compensated by using variable size unit cell elements. The main beam is directed to the desired direction (theta = 0 degrees). A 169-elements reflectarray is illuminated by a pyramidal horn antenna which is located at F = 148 mm. The designed reflectarray antenna is simulated and fabricated to validate the design method. The measurements are made in a shielded anechoic chamber. The simulation and measurement results are in agreement with each other. The maximum gain value is 22.93 dBi at 12 GHz, 1-dB gain bandwidth is 18% between 10 and 12 GHz and 3-dB gain bandwidth is 24% between 9.4 and 12 GHz.