[nearbuy]

The author of the article makes some assumption about the distribution of alien animal sizes to come up with their estimate. But they use loose bounds, saying intelligent life can probably vary in size at least as much as the great apes (50kg - 160kg), but probably not to tens of millions of kilograms.

So it seems you need at least some guess of what the distribution might look like to quantify the likelihood of guessing correctly that you're in a high population country. But your guessed distribution doesn't have to be perfect. If you knew the exact distribution, you could compute the exact chance that a random individual is from a high pop country (just take the integral of the distribution). But if you don't know the exact distribution, you can abstract one level up, try to reason about the range of possible distributions there could be, and come up with a probability based on your more limited knowledge. At least, that's what I understood from the article.

For the actual calculations, the author links to them in the article, but I haven't looked at them myself.