The box is fitted mainly to suit the fancier and this includes what space he has available. It might be worth bearing in mind that some birds do have a preference for a certain type and size of box and their placement in the breeding box and I wonder at times whether this is based on the original box they were brought up in. It's something to bear in mind when you buy stock in. Take a look at the birds first home. A strange box might not appeal to newly purchased stock.
Perches are another subject which vary from aviary to aviary.
Some fanciers feel that square perches are far better than round and they come up with various answers why they use them. Fair enough, but I have seen birds breeding great on oval ones and hazel branches which vary in shape and sizes. Here again I doubt whether there is scientific proof that one type of perch is better than another. It's something the fancier has come to a personal decision upon, and that is fair enough. I have seen birds mate on the floor and at one time, when we used a glass door on the nest box, some were seen to mate in the nest box itself.