In Pakistan, many local bird breeders use  traditional style of pots for birds to breed. These round bottom, clay made utensils or pots were and still are being used for this purpose. The size of the pot changes with the size of the parrot. I wont say, they don't work, 'cause I have seen birds like budgerigars, Rosella, Alexandrines, Cockatiels breeding in them without hesitation. 

The problem with this style is that the young ones, in curiosity to see outside the pot, fall from the hole easily as the hole is not very far away from them and secondly, in winter season, the clay becomes too cold which results in death of young either after birth or before it.

But as the world is becoming a global village, we have easy access to internet and lots of information worldwide, which has given us the idea of using the wooden boxes for the breeding purpose. The size of wooden boxes vary for different sizes of birds too.

Here I have given the sizes that are being used in my farm and I don't find any problem with these sizes. 

(All sizes are given in length x breadth x width)