From lip reading I think she's saying 'Oh my god, so fine!'
OMG, I just saw the video and wasn't even looking for it! The X factor bits were so funny! Katy actually says nothing, she just moves her mouth! And yes, when you see the whole thing it's obvious she refers to Cheryl's body parts. I first thought it's 'she's so tall' but it didn't make sense with Cheryl. It's something about her bum, haha. You can also see her huge heels and Katy's bare feet in the video.