This is a classic guesstimate question where you need to think aloud. And so first off you round the U.S. population to 300 million people (it's actually about 315 million but rounding will be much easier and your interviewer will not score you lower for rounding). Then estimate how many people eat pizza. A decent educated guess is two out of every three people, or 200 million. Now let's say the average pizza-eating person eats pizza twice a month, and eats two slices at a time. That's four slices a month.
If the average slice of pizza is perhaps six inches at the base and 10 inches long, then the slice is 30 square inches of pizza. So, four pizza slices would be 120 square inches (30 times 4). Since one square foot equals 144 square inches (12 times 12), let's assume that each person who eats pizza eats one square foot per month.
Since there are 200 million pizza-eating Americans, 200 million square feet of pizza are consumed in the U.S. each month. To summarize: 300 million people in America, 200 million eat pizza, average slice of pizza is six inches at the base and 10 inches long or 30 square inches, average American eats four slices of pizza a month, four pieces times 30 square inches equals 120 square inches (one square foot is 144 square inches), so let's assume one square foot per person, and thus one square foot times 200 million people equals 200 million square feet of pizza a month.