The minimum cage size recommended for a cockatiel is at least twice the bird's wingspan, translating to dimensions of approximately 20 inches in width, 30 inches in depth, and 30 inches in height. This size ensures enough space for the bird to move freely and exercise its wings. Providing a larger cage is even more beneficial, as it allows for a richer environment with multiple perches of varying heights and materials, which contributes to the bird's physical health and mental stimulation.