Guessing the answer without knowing the question

Ian Ayres asks a question at Freakonomics. Which of the following is the correct answer? 4π, 8π, 16, 16π or 32π square inches?

No, I didn't forget the question. But it's possible to make a reasonable guess by trying to reverse-engineer the question.

(Don't read the comments. They're full of people who didn't get it.)