When I first became involved with Quaker Parrots, the wisdom of the day was to purchase a cage that is no smaller than 18×18×18. I believe the thinking was that this size would be sufficient to allow a Quaker to stretch and flap its wings. Of course, that would depend on how many toys and other items are in the cage, but that was the general idea. After living with these birds for a number of years, I agree that this size provides a sufficient home for a Quaker and personally would never consider a cage that was smaller than this.
Read the full article: The Best Cage Size for Quaker Parrots