These are some of the resolution options of the 2015 27-inch iMac with Retina 5K display. The standard Display settings in El Capitan give you five options.
Larger Text – 1600 x 900 (yellow)
Default – 2560 x 1440 (red)
More Space – 3200 x 1800 (blue)
In order to access the full 5120 x 2880 (green) resolution you need to hold the OPTION key on they keyboard while you click on SCALED in the Display settings.