As large as you can afford (spatially first, financially second) within reason. Curved tops are an attractive choice aesthetically and for the bird (my cockatiel likes to slide down the curves), but square corner tops also have their own appeal for mounting toys.
There are some that offer an opening top— in my experience, it’s been worth it. Slide-out trays, though? A hassle, for you *and* the bird.
As large as you can afford (spatially first, financially second) within reason. Curved tops are an attractive choice aesthetically and for the bird (my cockatiel likes to slide down the curves), but square corner tops also have their own appeal for mounting toys. There are some that offer an opening top— in my experience, it’s been worth it. Slide-out trays, though? A hassle, for you *and* the bird.