we always tell people 'get the biggest cage you can afford'. I know that's not much help, but alexes are such acrobatic flyers that it's a kindness to give them as much space as possible. If your birds are out of their cages for quite along time each day, then a large cage isn't all that necessary. However, if they spend most of their day in the cage, then do try to get the biggest one you can.
When i was deciding which cage to get, i found one that is 2m wide, 2m tall and a metre deep. It has a central divider which i can slide in or out. That makes it very useful for cleaning and changing toys around. I can put the birds in one side while i take care of the other, then swap. I've kept the middle part of the cage clear of hanging things so the birds can actually fly from one perch to the other. They also have plenty of space to just flap their wings for exercise and they do this very often.
I like the big cage so much that i bought a second one for my corella. She doesn't fly as much as the alexes do, but she climbs and jumps and hangs upside down and uses every particle of space in her cage. Again, i'm glad i got her such a big one.
In the end, all you can do is your best. If your birds are healthy and get plenty of exercise, then you're doing pretty well. I look forward to watching them both grow up, especially little heero (i have a soft spot for alexandrines, which are the most beautiful of birds).
