I'd say a minimum of 3x3 ft.
They are active guys, and bigger than you expect.
And, Wendy's babies are big babies. Alice is a big girl compared to some of my friends Alex's.
Heres Alice and her brother on my friend, so you can get an idea of how big these guys run. Lisa (my friend) isnt a little gal, she's 5'8"
Or on the shoulder of my other friend...
Notice, both of em are big. Thats Alice in the front BTW LOL.
I have a pic somewhere of Alice and Buddy side by side, she's almost double his body length. I'll see if I can find it and add it.
Oh, and another plus for Wendy's birds... she raises them around a bunch of other species, so they take to other birds really well. Alice can safely play with just about anyone who will safely play with her. She is sweet as can be with the little IRN's. Now, Prinny may end up like Mazzie and hate other birds, but if she isnt like that... she'd have a playmate.
Another thing is toys... before you ever get your baby home stock up on chewable toys. Lots of wood. Alice goes through at LEAST a macaw sized wooden toy a week. She is a chewer, her brother is the same way... and I've heard that her parents are pretty darn good too!
Athena