Apple currently offers the iMac with two display types for each size. The 21.5-inch iMac comes with either a standard 21.5-inch display using 1920 by 1080 resolution, or a 21.5-inch Retina 4K display with a 4096 by 2304 resolution. The 27-inch iMac is only available with a 27-inch Retina 5K display using a 5120 by 2880 resolution.