Let me just say that I can sympathize. It's a very awkward position to be in when people ask you those types of things. I always went with, "Wow. That's a really personal question. What's your boyfriend's penis like?"
I was a teenager at the time... so I can't exactly recommend the above as polite. It's more like retaliatory rudeness. And, luckily, no one ever answered me back, they just kinda laughed and changed the subject.
Instead, I'd recommend a look. A surprised, and almost horrified look. Then compose yourself. Say in a serious tone, "That's a very personal question." And beandip the heck out of it.
ETA: I also love Tilt Fairy's idea. "I don't kiss and tell" is a wonderful thing.