Cockatiels are great beginner birds ! I think the size of the cage depends on how much time it is going to spend outside. If you have a job and your not at home most of the day then I think a large cage would be nice for them to entertain themselves in, if you are a house wife or do work from home and are able to take them out the whole day then you could get a cage slightly smaller. Cockatiels, like cockatoos show they're emotions and feelings by theyre crests so once you get the idea on which way or direction the crest is or moving it is easier to tell when your tiel is going to bite, is happy, or in a bad mood