There are evidently several different types of parrot species with varying wing spans. Accordingly, there isn't an exact figure for how big a parrot cage should be; the best size of cage entirely depends on the wingspan of a species. One rule of thumb is that the horizontal length of a cage should be at least 1 1/2 to 2 times the wingspan of its parrot. However, when researching the wingspan of your parrot's breed, remember that flight obstructions such as perches and swings reduce the space available for adequate wing size.