Cage size depends on how much time the birds spend inside their cage versus outside their cage and how much "stuff" (accessories) you put in the cage. If birds spend much of their day outside their cage and only nap, sleep, and eat inside, the cage size can be smaller than for a bird(s) that spend all their time inside their cage. Birds that spend all their time inside their cage need lots of room. Also, the number of items you include in the cage takes up space, so in selecting cage size, determine how many toys, feed/water dishes will be necessary, perches, etc., and number of birds. Birds must be able to vigorously flap their wings inside their cage without any part of their bodies touching anything inside the cage...this would be the minimum cage size. Also take into consideration the personalities of the birds involved, their sexes, etc. Some birds require more room than others in order to live peacefully together. Bigger is always better when it comes to housing birds.