I've got four, and I'm building another.
My first two came years ago when we had cockatiels. The first one is about 2'x2'x3'. It housed Sundown, our first bird. We bought Daystar, our second bird and purchased a larger cage, about 2½'x3'x4'.
The third one came with my glider, Sasha. It is the smallest cage, about 1½'x1½'x2½' ... and it reeks of neglect and pot.
My fourth one is really more of a long term loan. It's previous owners were all ferrets, so it got a good scrubbing. It measures about 3'x3'x4½'.
I've been wanting a walk-in aviary, but wanted someone to put in there as an excuse to build one first. I picked up Sasha, so now I have the excuse!
It will be at least 6'x8'x6'.