This question already has an answer here:
Randall Munroe posted an interesting what-if in which he claims that you can't use moonlight to light a fire, because it is impossible for reflected light to raise the temperature of an object higher than that of the reflecting surface, which of course is obviously false.
I continued thinking about the problem, and read a couple of other questions on the topic, and I'm not asking (as those questions do) whether it is possible, as it clearly is, but how you could do it.
So, here is the scenario: You are in an open plane with no obstructions (trees, clouds, buildings etc) between you and the sky. The moon is full. You can magically suspend a lens of any size and shape anywhere between you and the moon (but any sunlight that hits it mysteriously vanishes - moonlight only!). You are attempting to set fire to a piece of paper.
How big would the lens need to be to achieve this?