Is there a particular reason you're looking to cover up the existing carpet? I mean, sure, it's not exactly a luxurious shag rug in there, but it's not that bad.
I was there for Horizons, and I'd guess the room size at about 10'x16', but that's just a *very* rough estimate as I wasn't really thinking about it at the time. If what you're looking for is just a general idea, that should be close enough. I personally wouldn't recommend trying to buy carpet in advance, it just strikes me as a really bad idea. On the other hand, if you have some sort of free source or are just looking to know about how much it's going to cost, well, go ahead.
If you need a really exact estimate, I'd give building services a call tomorrow. They should be able to give you a much better idea, as they've actually got floor plans and such lying around.
Best of luck!
Edward
*disclaimer* If I fail at eyeball measurements, pleas don't shoot me!