Cockatiels need a minimum cage size of 16 inches wide and 16 inches deep to be able to open and flap their wings inside their cages. Because cockatiels often have night-frights (a startled awakening in the middle of the night), they need extra wing-span room or they may damage their wings severely in flight.
To help your pet cockatiel see during a night-fright, place the cage near an electrical outlet and plug in a night light. Then leave the cage cover slightly higher on that side of the cage to let a small amount of light in.