How big of a cage a bird needs is determined largely by its size and breed as well as the overall general nature of the bird. Obviously, a large bird is going to require a large cage, but don’t assume that just because the bird is tiny, they will only require a small cage. Some small birds require a lot of space to fly about. When deciding on what cage to by, ask questions like, “Do they need to fly about often?”, are they “energetic?” If they need room to spread their wings and fly around inside the cage, make sure you buy a large cage. Also, the bar spacing is an important factor in keeping your bird caged.
You should also get a secondary cage for transporting the bird to the vet or for travel. Like buying a house, the cage is just where the bird will stay while remaining as your pet, but they will need furnishings for the cage as well such as perches. The kind of perch and where you place it in the cage will be determined by the kind of bird you get. Though, make sure the bird can climb from perch to perch and that water and food dishes are not placed directly underneath a perch since they tend to poop when they land. Also make sure the perch is not placed so as to not get in the way of the bird flying.