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Did the math, lot of it, on the dimensions of the pylon in Malleus that was half a kilometer high and a quarter kilometer square at the base. Just something to pass the time. Where would we put that the pylon had the cubic volume of over 51,300 Olympic size swimming pools? That's assuming it was a cube.
According to various other sources, most Ordo Xenos Inquisitors know little-to-nothing about the Eldar, and those that do have gone significantly out of their way (at great personal risk) to study them. They're not called "the enigmatic Eldar" for nothing.
Funny thing—Dark Heresy, set in the next sector over, explicitly says Eldar are pretty scarce there. It's likely Eisenhorn doesn't _need_ to know as much about them as about more local races.
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How well does it match the trope?