Airspace; A Cage through the Eyes of a Parrot. Volume as a measurement of a parrot's cage tends to tell a clearer story of the correct size of choice. After all, it's the volume of that space our parrot will inhabit.
Our bird room is 20' X 25' by 8 and a half feet, or about 4250 cubic feet of space. Unfortunately, I'm 5' 6" so I'm not utilizing that top 1000 or so cubic feet. Much like how some parrots do not utilize the bottom 1/3 of their cage much. It's there, but doesn't really count. Since we've added a flock I am happy to say that all 4000 cubic feet in the bird room gets used at some time during the day.
In this graphic of a medium-large sized cage there is 60 cubic feet as an interior space. But only 40 of that will be used with enjoyment and for lengths of time. If you've a macaw, then at any given point approximately 3 cubic feet of space is used by the parrot. Increase that to 6 cubic feet of space when he's got his wings out stretched. (I'm leaving some space for movement in this math). Add toys, bowls, ropes, a bridge and the volume can shrink pretty fast.
Real Estate folks refer to this type of space as "useable square footage" or "liveable footage", I prefer the later since it really does refer to the comfort factor of space. I wrote a cage article not too long ago challenging cage shoppers to stick their head in the empty cage of their choice for understanding while shopping, for this very perspective.
Volume is everything, it's easier to see a parrot's perspective through volume rather than width, height and depth. Consider the main volume area when shopping. And when you toy shop consider how to create interest to magnify that volume usage. Get the most fun out of every cubic foot and your companion parrot will love every cubic inch.