QUOTE (Cacophony @ Oct 19 2009, 11:49 AM)

The basic rule of thumb seems to be no smaller than 18"x18"x18" on the inside of the cage. Unless you're talking a sleeping only cage... =) We use one of those with Kiko so that she can be in a quiet room when it's bedtime. And so we can lock the cats out. I think I have one size smaller than the one you've got from the looks of things - mine doesn't have the square play top - and it seems to be plenty of room for our girlie especially if you add a few perches OUTSIDE the cage to make the usable space bigger. Kiko is perched a half a foot from my computer screen on the end of a 2.5' perch put there so she can come over and be social.
I do have a rope perch outside of the cage that Sammy loves. I think this one might be 18 x 18 so I am thinking about eventually getting him a new cage. We will see how things go. I found a couple online that aren't too bad in price.