On Bird Cages

I used to work in a pet store. I'll admit that's where some of the early critters came from. Our store policy had what I still find to be a very stupid line about birds. If a customer asked,"How big does a bird's cage have to be?" The answer was "big enough that it's wings don't touch the sides when spread." My answer which was deemed incorrect was what kind of bird? In my mind when I think bird I instantly think of those horrible small finch cages we used to sell, maybe 12" wide. According to store policy that was a fine finchcage. I always thought of it as slow torture for a bird set up to fly everywhere. That said I have a cockatiel named Max. Someone dumped it at that store going on 5 years ago. She was hand tame and friendly and came with a cage maybe 18" wide. I've since upgraded her twice. She now lives in a 36" by 24" by 30" flight cage. Technically it is a flight cage for smaller birds, but she hasn't complained about the space. So far I haven't experienced any negative behaviors even though I don't get her out of the cage as often as I should. I always feel guilty about not getting her out. A few months ago I was at an acquaintance's house and they had two unfriendly cockatiels. Both were in cages that when spread their wings would just fit. They wouldn't be able to flap them, but technically those cages were considered big enough. I suddenly stopped feeling guilty about my cockatiel. No, she doesn't get out as often as I'd like, but she has plenty of space to crawl around, I don't clip her wings and when she does get out she is still as nice as can be.