Context: You have no idea how to size the particular problem because you have never interacted with said topic
Example: Estimate the weight of the Eiffel tower, by estimating the number of blocks the Eiffel tower takes up.
We know the tower is big, made of steel, can you just ask how tall it is? (explaining you would use the height and number of blocks it takes up in an effort to calculate the total mass and calculate that by the weight of the materials.
Similiarly, we know the population of the United States, but even as an American I wouldnt know how big the country is in area. Would it be professional to say all the facts you did know, but ask how big the area in question is?