Mine is 11 x 14 x 8... and that's in feet.

I'd say minimum, for one cockatiel, should be 24" x 24" but that's just me. The best cages tend to be ones that have more horizontal space vs vertical space, although since cockatiels do have long tails, they need *some* vertical space, too. I currently have 3 cages setup, although none house any tiels. One cage is the HQ Flight Cage. I like the fact that it's a large cage, it was cheap (
$100 - even though I got it second hand - brand new), it's got a flimsy plastic tray (
in most other cases, this *may* be a downside rather than an upside, because the cage will last longer than the tray - however, the inhabitant is known for falling, so in *her* case, I'd rather have a "soft" plastic landing vs hard metal), it's easy to clean, and it has two doors. I do, and do not like the two doors, and I can't say I'd like, or dislike the same version of the cage with one door. (
made by A&E)
I also have a YML WI32R cage (
and a larger version, not in use, the WI40R), it's got a large door, but the bird who uses the cage doesn't need such a big door... then again, the cage was made for species whom are 2-3 times larger than the bird who's using it (
a mitred conure, at 230 grams - the previous occupant was a quaker who probably weighed about 90+ grams). I don't care for the cage door locking system, but only because I occasionally get attacked, and a different locking system may make it easier to close the cage door quicker. I don't like the gap's between the cage grate and the tray, either, nor as flimsy as the wire is in that spot, but that aside, it's still a good cage.
The last cage is a smaller one, a 17" x 17" which I prefer to use for a travel cage or hospital cage, although currently houses two budgies (
one of which isn't feeling well atm and is getting meds). They did have a larger cage but they didn't come with one, so it's temporary housing, and they enjoy coming out of the cage for some exercise, as well. Neither one are clipped. This is one of the cages that has a large door and a smaller door, so the large door doesn't have to "get in the way," so to speak. It's just a basic square cage, nothing fancy, which is what I prefer.