I don't remember the size exactly (you'd have to get ardenbird to tell you that) but I do remember they were bigger than we first thought we needed. We got what we thought would be plenty big, brought it home, and the first thing that happened was our cat went in and arched her back up and hit the top of the box. We knew right then that it wouldn't pass. So back to the store to exchange for a bigger one. The one we ended up with said it was for medium sized dogs.
The Virgin cargo department said - in fact strongly suggested - that we come in the day before and show them our cage, with cat in it, so they could tell if it was acceptable. We did that and all was fine. But someone else on the day we flew out did not come by in advance and they were running out to the pet stores at the last minute trying to get a bigger cage. We did not see them at the reception center in London, so they might have missed the flight. I don't know if Continental pre-inspects too, but I'd recommend it.