I recomend keeping your pet cockatiels in a cage no less than 400mm x400mm x400mm high

Most of my birds are bred in suspended aviaries which are 1800mm x 860mm x 900mm high. After the pairs have finished breeding, they are put back in one of the larger conventional aviaries for a rest.
 

In the photo above, you can see two large conventional aviaries with six breeding suspendeds in the back left corner of the yard.